[Swift 6] Add Swift 6 testing for SharedSwift (#14557)

Author: ncooke3

.github/workflows/shared-swift.yml

@@ -27,8 +27,13 @@ jobs:
         build-env:
           - os: macos-14
             xcode: Xcode_15.2
+            swift_version: 5.9
           - os: macos-15
             xcode: Xcode_16.2
+            swift_version: 5.9
+          # - os: macos-15
+          #   xcode: Xcode_16.2
+          #   swift_version: 6.0
     runs-on: ${{ matrix.build-env.os }}
     steps:
     - uses: actions/checkout@v4
@@ -37,6 +42,8 @@ jobs:
       run: scripts/setup_bundler.sh
     - name: Xcode
       run: sudo xcode-select -s /Applications/${{ matrix.build-env.xcode }}.app/Contents/Developer
+    - name: Set Swift swift_version
+      run: sed -i "" "s/s.swift_version[[:space:]]*=[[:space:]]*'5.9'/s.swift_version = '${{ matrix.build-env.swift_version }}'/" FirebaseSharedSwift.podspec
     - name: Build and test
       run: scripts/third_party/travis/retry.sh scripts/pod_lib_lint.rb FirebaseSharedSwift.podspec --platforms=${{ matrix.target }}