Update build matrix in GitHub Actions workflow

Removed older Smalltalk versions from the build matrix.

Author: codeZeilen

.github/workflows/main.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       matrix:
         os: [ ubuntu-latest ]
-        smalltalk: [ Squeak64-trunk, Squeak64-6.0, Squeak64-5.3, Squeak32-5.2, Squeak32-5.1 ]
+        smalltalk: [ Squeak64-trunk, Squeak64-6.0, Squeak64-5.3 ]
         # Since git/s does not deal with platforms really,
         # only run the latest versions on Windows and macOS.
         include: