Centralize the officially supported bazel version

Add a .bazelversion file so that workflows and contributors can
automatically use the same supported/tested version of bazel.

Author: jdcormie

.bazelversion

@@ -0,0 +1 @@
+8.7.0

.github/workflows/testing.yml

@@ -80,8 +80,6 @@ jobs:
     strategy:
       matrix:
         bzlmod: [true, false]
-    env:
-      USE_BAZEL_VERSION: 8.7.0
 
     steps:
     - uses: actions/checkout@v4

COMPILING.md

@@ -150,3 +150,19 @@ $ANDROID_HOME/cmdline-tools/latest/bin/sdkmanager --licenses
 
 # Add 'export ANDROID_HOME=$HOME/Android/Sdk' to your .bashrc or equivalent
 ```
+
+Building with Bazel
+===================
+
+grpc-java can also be built using [Bazel](https://bazel.build/).
+
+First [install bazelisk](https://github.com/bazelbuild/bazelisk#installation)
+(unlike Gradle, Bazel's wrapper isn't checked-in to the grpc-java repository).
+
+To build the grpc-java library:
+```
+$ bazelisk build //...
+```
+
+You cannot run the tests from Bazel at this time.
+