Update docs for protoc minimum installed version (#11006)

* bumps `protoc` minimum version from 3.12 -> 3.15

* Minor: Fix link for Protoc installation

* minor: removes reference to 3.12

datafusion/proto/CONTRIBUTING.md

@@ -29,4 +29,4 @@ valid installation of [protoc] (see [installation instructions] for details).
 ```
 
 [protoc]: https://github.com/protocolbuffers/protobuf#protocol-compiler-installation
-[installation instructions]: https://datafusion.apache.org/contributor-guide/#protoc-installation
+[installation instructions]: https://datafusion.apache.org/contributor-guide/getting_started.html#protoc-installation

docs/source/contributor-guide/getting_started.md

@@ -50,11 +50,11 @@ $ pacman -S protobuf
 $ brew install protobuf
 ```
 
-You will want to verify the version installed is `3.12` or greater, which introduced support for explicit [field presence](https://github.com/protocolbuffers/protobuf/blob/v3.12.0/docs/field_presence.md). Older versions may fail to compile.
+You will want to verify the version installed is `3.15` or greater, which has support for explicit [field presence](https://github.com/protocolbuffers/protobuf/blob/v3.15.0/docs/field_presence.md). Older versions may fail to compile.
 
 ```shell
 $ protoc --version
-libprotoc 3.12.4
+libprotoc 3.15.0
 ```
 
 Alternatively a binary release can be downloaded from the [Release Page](https://github.com/protocolbuffers/protobuf/releases) or [built from source](https://github.com/protocolbuffers/protobuf/blob/main/src/README.md).