update macos version for runner (#9817)

Closes: https://github.com/neondatabase/neon/issues/9816

Run MacOs builds on `macos-15`.
As `pkg-config` is bundled in runner image, don't install it with `brew`

.github/workflows/neon_extra_builds.yml

@@ -38,7 +38,7 @@ jobs:
       contains(github.event.pull_request.labels.*.name, 'run-extra-build-*') ||
       github.ref_name == 'main'
     timeout-minutes: 90
-    runs-on: macos-14
+    runs-on: macos-15
 
     env:
       # Use release build only, to have less debug info around
@@ -52,7 +52,7 @@ jobs:
           submodules: true
 
       - name: Install macOS postgres dependencies
-        run: brew install flex bison openssl protobuf icu4c pkg-config
+        run: brew install flex bison openssl protobuf icu4c
 
       - name: Set pg 14 revision for caching
         id: pg_v14_rev