Allow jobs to upgrade to macos-15 (using -latest) (#16321)

# Objective

With [macos-15 becoming the new `-latest`
soon](https://github.com/github/roadmap/issues/986), there's probably no
particular reason left to pin a runner version here. May as well take
the upgrade as it rolls out.

Note that these are still m1-based, you'd have to request a larger
runner to get m2.

.github/workflows/ci.yml

@@ -72,8 +72,7 @@ jobs:
         run: cargo run -p ci -- lints
 
   miri:
-    # Explicitly use macOS 14 to take advantage of M1 chip.
-    runs-on: macos-14
+    runs-on: macos-latest
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@v4
@@ -254,8 +253,7 @@ jobs:
           echo 'You can find the extension here: https://marketplace.visualstudio.com/items?itemName=tekumara.typos-vscode'
 
   run-examples-macos-metal:
-    # Explicitly use macOS 14 to take advantage of M1 chip.
-    runs-on: macos-14
+    runs-on: macos-latest
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v4