ci: bump GOVERSION

GitHub Actions was configured to use Go 1.17, which is quite old. This commit
bumps it to Go 1.21, which is the older of the two currently-supported Go
versions.

This commit also adjusts the command-line for installing `golint` to be
compatible with more recent Go versions.

Keeping go.mod at 1.17 for now though, unless/until we actually need newer Go
language features. Since github.com/skeema/knownhosts is imported by a lot of
other Go packages, we don't want to force increases to that listed version
unnecessarily.

.github/workflows/tests.yml

@@ -1,7 +1,7 @@
 name: Tests
 on: [push, pull_request]
 env:
-  GOVERSION: "1.17"
+  GOVERSION: "1.21"
 jobs:
   test:
     name: Check code quality and run tests
@@ -20,7 +20,7 @@ jobs:
         run: test -z "$(gofmt -s -d *.go 2>&1)"
 
       - name: Run golint
-        run: go get -u golang.org/x/lint/golint && golint -set_exit_status
+        run: go get golang.org/x/lint/golint && go install golang.org/x/lint/golint && golint -set_exit_status
 
       - name: Run go vet
         run: go vet