chore: rename default branch to main (#851)

Author: Neenu1995

.github/release-please.yml

@@ -1,3 +1,3 @@
 releaseType: simple
 handleGHRelease: true
-primaryBranch: master
+primaryBranch: main

.github/workflows/ci.yaml

@@ -1,14 +1,14 @@
 on:
   push:
     branches:
-      - master
+      - main
   pull_request:
 name: ci
 jobs:
   build:
     runs-on: ubuntu-latest
     container: gcr.io/gapic-images/googleapis-bazel:20210105
-    # Dockerfile for this image: https://github.com/googleapis/googleapis-discovery/blob/master/Dockerfile
+    # Dockerfile for this image: https://github.com/googleapis/googleapis-discovery/blob/main/Dockerfile
     # If you update its version, please also update it below in
     # 'Cache Bazel files' - unfortunately it cannot accept variables at this
     # time.

README.md

@@ -1,4 +1,4 @@
-[![codecov](https://codecov.io/gh/googleapis/gapic-generator-java/branch/master/graph/badge.svg?token=3RUU37GX9U)](https://codecov.io/gh/googleapis/gapic-generator-java)
+[![codecov](https://codecov.io/gh/googleapis/gapic-generator-java/branch/main/graph/badge.svg?token=3RUU37GX9U)](https://codecov.io/gh/googleapis/gapic-generator-java)
 # API Client Generator for Java
 
 Generates a Java client library from protocol buffers.