CI: Drop ubuntu-20.04 jobs

Unfortunately, those older versions of gcc and clang are not in ubuntu-22.04

Author: solardiz

.github/workflows/ci.yml

@@ -94,20 +94,6 @@ jobs:
     - name: build check
       run: ci/run-build-and-tests.sh
 
-  gcc8-x86_64:
-    runs-on: ubuntu-20.04
-    env:
-      CC: gcc-8
-      TARGET: x86_64
-    steps:
-    - uses: actions/checkout@v5
-      with:
-        fetch-depth: 0
-    - name: install dependencies
-      run: ci/install-dependencies.sh
-    - name: build check
-      run: ci/run-build-and-tests.sh
-
   clang18-x86_64:
     runs-on: ubuntu-24.04
     env:
@@ -233,31 +219,3 @@ jobs:
       run: ci/install-dependencies.sh
     - name: build check
       run: ci/run-build-and-tests.sh
-
-  clang9-x86_64:
-    runs-on: ubuntu-20.04
-    env:
-      CC: clang-9
-      TARGET: x86_64
-    steps:
-    - uses: actions/checkout@v5
-      with:
-        fetch-depth: 0
-    - name: install dependencies
-      run: ci/install-dependencies.sh
-    - name: build check
-      run: ci/run-build-and-tests.sh
-
-  clang8-x86_64:
-    runs-on: ubuntu-20.04
-    env:
-      CC: clang-8
-      TARGET: x86_64
-    steps:
-    - uses: actions/checkout@v5
-      with:
-        fetch-depth: 0
-    - name: install dependencies
-      run: ci/install-dependencies.sh
-    - name: build check
-      run: ci/run-build-and-tests.sh