Drop official support for using Xcode 11 to build MoltenVK.

- Remove Xcode 11 build from GitHub CI.
- Upgrade primary CI builds to use Xcode 15.
- Leave MVK_XCODE_12 guards in place to allow devs to possibly continue to
  attempt to build existing MoltenVK code using Xcode 11, even though it's
  not officially supported. Such devs may have to add their own additional
  MVK_XCODE_12 guards for any Xcode 12 API features added after this change.

.github/workflows/CI.yml

@@ -15,21 +15,17 @@ jobs:
   build:
     strategy:
       matrix:
-        xcode: [ "14.3" ]
+        xcode: [ "15.0" ]
         platform: [ "all", "macos", "ios" ]
         os: [ "macos-13" ]
         upload_artifacts: [ true ]
-        # additional specific configurations
+
+        # Legacy configurations
         include:
-          # "Legacy" Xcode 12.5.1 & 11.7 macOS builds
           - xcode: "12.5.1"
             platform: "macos"
             os: "macos-11"
             upload_artifacts: false
-          - xcode: "11.7"
-            platform: "macos"
-            os: "macos-11"
-            upload_artifacts: false
       fail-fast: false
 
     name: 'MoltenVK (Xcode ${{ matrix.xcode }} - ${{ matrix.platform }})'

Docs/Whats_New.md

@@ -32,6 +32,7 @@ Released TBD
 - Support maximizing the concurrent executing compilation tasks via `MVKConfiguration::shouldMaximizeConcurrentCompilation`
 - Add support for `VK_PRESENT_MODE_IMMEDIATE_KHR` to macOS Cube demo.
 - Log more info about SPIR-V to MSL conversion errors.
+- Drop official support for using *Xcode 11* to build MoltenVK.