Adjust configuration and documents to reflect the renamed main branch.

RELNOTES=n/a
PiperOrigin-RevId: 492336825

Author: eamonnmcmanus

.github/workflows/ci.yml

@@ -3,10 +3,10 @@ name: CI
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
     branches:
-      - master
+      - main
 
 jobs:
   test:

README.md

@@ -24,7 +24,7 @@ License
     See the License for the specific language governing permissions and
     limitations under the License.
 
-[ci-shield]: https://github.com/google/compile-testing/actions/workflows/ci.yml/badge.svg?branch=master
+[ci-shield]: https://github.com/google/compile-testing/actions/workflows/ci.yml/badge.svg?branch=main
 [ci-link]: https://github.com/google/compile-testing/actions
 [maven-shield]: https://img.shields.io/maven-central/v/com.google.testing.compile/compile-testing.png
 [maven-link]: https://search.maven.org/artifact/com.google.testing.compile/compile-testing