chore(ci): Use Xcode 15.2 (#3924)

Fixes these warnings:


https://github.com/firezone/firezone/actions/runs/8149132180/job/22273278837?pr=3906#step:6:592

.github/workflows/_swift.yml

@@ -14,10 +14,12 @@ jobs:
           - sdk: macosx
             runs-on: macos-14
             platform: macOS
+            xcode: '15.2'
             destination: platform=macOS
           - sdk: iphoneos
             runs-on: macos-14
             platform: iOS
+            xcode: '15.2'
             destination: generic/platform=iOS
     permissions:
       contents: read
@@ -95,6 +97,9 @@ jobs:
           # Needed because `productbuild` doesn't support picking this up automatically like Xcode does
           INSTALLER_CODE_SIGN_IDENTITY: "3rd Party Mac Developer Installer: Firezone, Inc. (47R2M6779T)"
         run: |
+          # Use the same Xcode version as development
+          sudo xcode-select -s /Applications/Xcode_${{ matrix.xcode }}.app
+
           # Copy xcconfig
           cp Firezone/xcconfig/release.xcconfig Firezone/xcconfig/config.xcconfig