chore(workflows): update to macos-15 runners

Upgrade from `macos-13` and `macos-14` runners to `macos-15` and 
`macos-15-intel` runners.

Previously we were still using `macos-13`, as the default free tier were
Intel-based. From macos-14 and later the default free tier is ARM-based.

.github/workflows/_prepare.yml

@@ -18,7 +18,7 @@ on:
 
 jobs:
   builder-sha:
-    runs-on: "macos-13"
+    runs-on: "macos-15"
     outputs:
       ref: ${{ steps.ref.outputs.ref }}
       sha: ${{ steps.sha.outputs.sha }}
@@ -54,8 +54,8 @@ jobs:
     strategy:
       matrix:
         os:
-          - macos-13 # Only macos-13 and earlier are x86_64.
-          - macos-14 # Only macos-14 and later are ARM64.
+          - macos-15
+          - macos-15-intel
     runs-on: ${{ matrix.os }}
     steps:
       - name: Cache emacs-builder (${{ runner.arch }})