ci: Replace github macos 12 with macos 15

macos 12 is deprecated

Author: mkj

.github/workflows/build.yml

@@ -73,8 +73,8 @@ jobs:
             # .../ranlib: file: libtomcrypt.a(cbc_setiv.o) has no symbols
             ranlib: ranlib -no_warning_for_no_symbols
 
-          - name: macos 12
-            os: macos-12
+          - name: macos 15
+            os: macos-15
             cc: clang
             # OS X says daemon() and utmp are deprecated.
             # OS X tests for undefined TARGET_OS_EMBEDDED in libc headers