Remove support for retired macos-13 (#59)

Author: GlassOfWhiskey

.github/workflows/test.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [macos-13, macos-15-intel]
+        os: [macos-15-intel]
     name: A job to perform a self-test on this Github Action
     steps:
       # To use this repository's private action,

README.md

@@ -13,7 +13,6 @@ I intend this action to be kept as simple as possible:
 
 ## Currently supported public runner images
 
-- `macos-13`
 - `macos-15`
 - `macos-15-large`
 - `macos-15-intel`