ci: upgrade from Ubuntu 20.04 to 22.04

CI was complaining about a too-old Ubuntu version. So let's upgrade. We
should be good for the next 2 years or so.

I upgraded to 22.04 (and not 24.04) because older LLVM versions (such as
LLVM 16) aren't available from apt.llvm.org for this newer Ubuntu
version.

Author: aykevl

.github/workflows/test.yml

@@ -31,7 +31,7 @@ jobs:
         run:
           go test -v
   test-linux:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-22.04
     strategy:
       matrix:
         llvm: [14, 15, 16, 17, 18, 19]
@@ -44,7 +44,7 @@ jobs:
           go-version: '1.22'
       - name: Install LLVM
         run: |
-          echo 'deb http://apt.llvm.org/focal/ llvm-toolchain-focal-${{ matrix.llvm }} main' | sudo tee /etc/apt/sources.list.d/llvm.list
+          echo 'deb http://apt.llvm.org/jammy/ llvm-toolchain-jammy-${{ matrix.llvm }} main' | sudo tee /etc/apt/sources.list.d/llvm.list
           wget -O - https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
           sudo apt-get update
           sudo apt-get install --no-install-recommends llvm-${{ matrix.llvm }}-dev