Fix failing Clang builds on GitHub Actions (#79, thanks @striezel)

A while ago the 'ubuntu-latest' image moved from Ubuntu 22.04 to
Ubuntu 24.04. See <actions/runner-images#10636>.
However, Ubuntu 24.04 does not have packages for Clang 11 and 12
anymore, thus the Clang jobs failed when they tried to install
the corresponding compilers. Therefore, the workflow is changed
to use 'ubuntu-22.04' instead of 'ubuntu-latest'.

Furthermore, jobs for Clang 16 (the first major version of Clang
to support `std::expected`) and Clang 19 (latest Clang version
available on Ubuntu 24.04) are added.

Author: striezel

.github/workflows/ci.yml

@@ -47,9 +47,17 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        version: [11, 12]
+        include:
+          - version: 11
+            os: 'ubuntu-22.04'
+          - version: 12
+            os: 'ubuntu-22.04'
+          - version: 16
+            os: 'ubuntu-24.04'
+          - version: 19
+            os: 'ubuntu-24.04'
 
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.os }}
 
     steps:
       - uses: actions/checkout@v4