Update MacOS Intel runners to MacOS 15 (#286)

Github is getting rid of the MacOS 13 Intel runners. They added
a MacOS 15 Intel runner that will be around til August 2027.

Author: SeanTAllen

.github/actionlint.yaml

@@ -2,3 +2,4 @@
 self-hosted-runner:
   labels:
     - windows-11-arm
+    - macos-15-intel

.github/workflows/breakage-against-macos-x86-ponyc-latest.yml

@@ -10,7 +10,7 @@ permissions:
 jobs:
   macos:
     name: Verify main against the latest ponyc on macOS
-    runs-on: macos-13
+    runs-on: macos-15-intel
     steps:
       - uses: actions/checkout@v4.1.1
       - name: install ponyc

.github/workflows/nightlies.yml

@@ -129,7 +129,7 @@ jobs:
 
   x86-64-apple-darwin-nightly:
     name: Build and upload x86-64-apple-darwin-nightly to Cloudsmith
-    runs-on: macos-13
+    runs-on: macos-15-intel
     steps:
       - uses: actions/checkout@v4.1.1
       - name: install ponyc

.github/workflows/pr.yml

@@ -96,7 +96,7 @@ jobs:
 
   x86_macos:
     name: x86_64 MacOS
-    runs-on: macos-13
+    runs-on: macos-15-intel
     steps:
       - uses: actions/checkout@v4.1.1
       - name: install ponyc

.github/workflows/release.yml

@@ -74,7 +74,7 @@ jobs:
 
   x86-64-apple-darwin-release:
     name: Build and upload x86-64-apple-darwin-release to Cloudsmith
-    runs-on: macos-13
+    runs-on: macos-15-intel
     needs:
       - pre-artefact-creation
     steps: