chore: adjust build matrix

- Remove all 32-bit builds of Pharo. They are usually identical to the
  64-bit builds but add considerable overhead.
- Remove 'Pharo64-stable' builds in favor of explicit inclusion of the
  stable version. We had issues with VM segfaults when using 'stable'.

.github/workflows/main.yml

@@ -26,10 +26,6 @@ jobs:
           - Squeak64-5.3
           - Squeak64-5.2
           - Squeak64-5.1
-          - Squeak32-trunk
-          - Squeak32-6.0
-          - Squeak32-4.5
-          - Pharo64-stable
           - Pharo64-alpha
           - Pharo64-12
           - Pharo64-11
@@ -39,9 +35,6 @@ jobs:
           - Pharo64-7.0
           - Pharo64-6.1
           - Pharo64-6.0
-          - Pharo32-stable
-          - Pharo32-alpha
-          - Pharo32-3.0
           - Moose64-11
           - Moose64-10
           - Moose64-9.0
@@ -52,19 +45,7 @@ jobs:
           - GemStone64-3.6.5
           - GToolkit64-release
         timeout-minutes: [ 15 ]
-        exclude: # exclude 32bit builds on macOS and GemStone builds on windows
-          - os: macos-latest
-            smalltalk: Squeak32-trunk
-          - os: macos-latest
-            smalltalk: Squeak32-6.0
-          - os: macos-latest
-            smalltalk: Squeak32-4.5
-          - os: macos-latest
-            smalltalk: Pharo32-stable
-          - os: macos-latest
-            smalltalk: Pharo32-alpha
-          - os: macos-latest
-            smalltalk: Pharo32-3.0
+        exclude: # exclude GemStone builds on windows; and some problematic builds of Pharo
           - os: windows-2019
             smalltalk: Pharo64-10
           - os: windows-2019