fix: use macos-15 runner for x86_64 cross-compilation

macos-13 runner is no longer supported by GitHub Actions.

Author: uk0

.github/workflows/release.yml

@@ -24,7 +24,7 @@ jobs:
             build_cli: true
 
           - platform: macos-x86_64
-            runner: macos-13
+            runner: macos-15
             target: x86_64-apple-darwin
             cli_name: ""
             gui_name: file-encryption-gui