GHA: drop EOL OSes

.github/workflows/raspbian.yml

@@ -15,7 +15,6 @@ jobs:
       fail-fast: false
       matrix:
         codename:
-          - buster
           - bullseye
 
     runs-on: ubuntu-22.04 # revert back to ubuntu-latest once that is 22.04 or later

.github/workflows/rpm.yml

@@ -36,18 +36,12 @@ jobs:
           - name: sles
             release: '15.4'
             subscription: true
-          - name: sles
-            release: '15.3'
-            subscription: true
           - name: sles
             release: '12.5'
             subscription: true
           - name: opensuse
             release: '15.4'
             subscription: false
-          - name: opensuse
-            release: '15.3'
-            subscription: false
 
     runs-on: ubuntu-latest