Update README.md to reflect changes

Author: erik-bershel

README.md

@@ -30,7 +30,7 @@ Archives are named `$engine-$version-$platform.tar.gz`.
 
 `platform` is one of:
 * `ubuntu-NN.NN`: built on the corresponding GitHub-hosted runner virtual environment
-* `macos-latest`: built on `macos-12`, the oldest `macos-amd64` available on GitHub-hosted runners.
+* `macos-latest`: built on `macos-13`, the oldest `macos-amd64` available on GitHub-hosted runners.
 * `macos-13-arm64`: built on `macos-14`, the oldest `macos-arm64` available on GitHub-hosted runners.
 * `windows-latest`: built on `windows-2019` (does not matter, it's only for repacking a JRuby archive, no actual build)