chore: update year in the license headers

[yoshi-automation]

This PR was generated using Autosynth. 🌈

Here's the log from Synthtool:

synthtool > Executing /tmpfs/src/git/autosynth/working_repo/synth.py.
synthtool > Ensuring dependencies.
synthtool > Pulling artman image.
latest: Pulling from googleapis/artman
Digest: sha256:12722f2ca3fbc3b53cc6aa5f0e569d7d221b46bd876a2136497089dec5e3634e
Status: Image is up to date for googleapis/artman:latest
synthtool > Cloning googleapis.
synthtool > Running generator for google/firestore/artman_firestore_v1.yaml.
synthtool > Generated code into /home/kbuilder/.cache/synthtool/googleapis/artman-genfiles/js/firestore-v1.
synthtool > Replaced '../../package.json' in dev/src/v1/firestore_client.js.
synthtool > Replaced 'module.exports.FirestoreClient = FirestoreClient' in dev/src/v1/index.js.
synthtool > Replaced 'new firestoreModule.v1.FirestoreClient\\(' in dev/test/gapic-v1.js.
synthtool > Replaced 'return this\\._innerApiCalls\\.listen\\(options\\);' in dev/src/v1/firestore_client.js.
.eslintignore
.eslintrc.yml
.github/CONTRIBUTING.md
.github/ISSUE_TEMPLATE/bug_report.md
.github/ISSUE_TEMPLATE/feature_request.md
.github/ISSUE_TEMPLATE/support_request.md
.jsdoc.js
.kokoro/common.cfg
.kokoro/continuous/node10/common.cfg
.kokoro/continuous/node10/test.cfg
.kokoro/continuous/node11/common.cfg
.kokoro/continuous/node11/test.cfg
.kokoro/continuous/node6/common.cfg
.kokoro/continuous/node6/test.cfg
.kokoro/continuous/node8/common.cfg
.kokoro/continuous/node8/docs.cfg
.kokoro/continuous/node8/lint.cfg
.kokoro/continuous/node8/samples-test.cfg
.kokoro/continuous/node8/system-test.cfg
.kokoro/continuous/node8/test.cfg
.kokoro/docs.sh
.kokoro/lint.sh
.kokoro/presubmit/node10/common.cfg
.kokoro/presubmit/node10/test.cfg
.kokoro/presubmit/node11/common.cfg
.kokoro/presubmit/node11/test.cfg
.kokoro/presubmit/node6/common.cfg
.kokoro/presubmit/node6/test.cfg
.kokoro/presubmit/node8/common.cfg
.kokoro/presubmit/node8/docs.cfg
.kokoro/presubmit/node8/lint.cfg
.kokoro/presubmit/node8/samples-test.cfg
.kokoro/presubmit/node8/system-test.cfg
.kokoro/presubmit/node8/test.cfg
.kokoro/presubmit/windows/common.cfg
.kokoro/presubmit/windows/test.cfg
.kokoro/publish.sh
.kokoro/release/publish.cfg
.kokoro/samples-test.sh
.kokoro/system-test.sh
.kokoro/test.bat
.kokoro/test.sh
.kokoro/trampoline.sh
.nycrc
.prettierignore
.prettierrc
CODE_OF_CONDUCT.md
LICENSE
codecov.yaml
renovate.json
synthtool > Replaced 'https:\\/\\/cloud\\.google\\.com[\\s\\*]*http:\\/\\/(.*)[\\s\\*]*\\)' in dev/src/v1/doc/google/protobuf/doc_timestamp.js.
synthtool > Replaced 'toISOString\\]' in dev/src/v1/doc/google/protobuf/doc_timestamp.js.
npm WARN deprecated text-encoding@0.6.4: no longer maintained

> grpc@1.18.0 install /tmpfs/src/git/autosynth/working_repo/node_modules/grpc
> node-pre-gyp install --fallback-to-build --library=static_library

node-pre-gyp WARN Using request for node-pre-gyp https download 
[grpc] Success: "/tmpfs/src/git/autosynth/working_repo/node_modules/grpc/src/node/extension_binary/node-v57-linux-x64-glibc/grpc_node.node" is installed via remote

> protobufjs@6.8.8 postinstall /tmpfs/src/git/autosynth/working_repo/node_modules/protobufjs
> node scripts/postinstall


> @google-cloud/firestore@0.20.0 prepare /tmpfs/src/git/autosynth/working_repo
> npm run compile


> @google-cloud/firestore@0.20.0 compile /tmpfs/src/git/autosynth/working_repo
> tsc -p . && cp -r dev/protos build && cp -r dev/test/fake-certificate.json build/test/fake-certificate.json && cp dev/src/v1beta1/firestore_client_config.json build/src/v1beta1/ && cp dev/src/v1/firestore_client_config.json build/src/v1/ && cp dev/conformance/test-definition.proto build/conformance && cp dev/conformance/test-suite.binproto build/conformance

npm notice created a lockfile as package-lock.json. You should commit this file.
npm WARN optional SKIPPING OPTIONAL DEPENDENCY: fsevents@1.2.6 (node_modules/fsevents):
npm WARN notsup SKIPPING OPTIONAL DEPENDENCY: Unsupported platform for fsevents@1.2.6: wanted {"os":"darwin","arch":"any"} (current: {"os":"linux","arch":"x64"})

added 1151 packages from 1303 contributors and audited 4537 packages in 32.135s
found 1 low severity vulnerability
  run `npm audit fix` to fix them, or `npm audit` for details

> @google-cloud/firestore@0.20.0 fix /tmpfs/src/git/autosynth/working_repo
> gts fix

synthtool > Cleaned up 2 temporary directories.
synthtool > Wrote metadata to synth.metadata.