Disable Bazel mixed versions on main and PRs

Author: lhoguin

.github/workflows/test-mixed-versions.yaml

@@ -2,7 +2,6 @@ name: Test Mixed Version Clusters
 on:
   push:
     branches:
-    - main
     - v4.0.x
     - v3.13.x
     - bump-otp-*
@@ -21,7 +20,6 @@ on:
     - '*.bzl'
     - '*.bazel'
     - .github/workflows/test-mixed-versions.yaml
-  pull_request:
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
   cancel-in-progress: true