[sim] Remove unmaintained Gazebo support (#4736)

.github/workflows/gazebo.yml

@@ -1,19 +0,0 @@
-name: Gazebo
-
-on: [pull_request, push]
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
-  cancel-in-progress: true
-
-jobs:
-  build:
-    name: "Build"
-    runs-on: ubuntu-22.04
-    container: wpilib/gazebo-ubuntu:22.04
-    steps:
-      - uses: actions/checkout@v3
-        with:
-          fetch-depth: 0
-      - name: Build with Gradle
-        run: ./gradlew simulation:frc_gazebo_plugins:build simulation:halsim_gazebo:build -PbuildServer -PforceGazebo