Update default branch from master to main in cmake.yml

The `pr-base-branch` value in the `cmake.yml` file was updated
from `master` to `main`. This change aligns with modern
branch naming conventions that favor the term `main` over
`master`.

Author: JunaMeinhold

.github/workflows/cmake.yml

@@ -28,5 +28,5 @@ jobs:
       lib-name: "zlib"
       lib-version: "v1.3.1"
       pr-branch: zlib-artifacts-branch
-      pr-base-branch: master
+      pr-base-branch: main
       pr-reviewers: JunaMeinhold