Move CI to Ubuntu 24.04 (#642)

Currently in beta (see https://github.com/actions/runner-images) but
should be stable enough for us.

Author: adamchainz

.github/workflows/main.yml

@@ -13,7 +13,7 @@ concurrency:
 jobs:
   tests:
     name: Python ${{ matrix.python-version }}
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
 
     strategy:
       matrix:
@@ -50,7 +50,7 @@ jobs:
 
   coverage:
     name: Coverage
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     needs: tests
     steps:
       - uses: actions/checkout@v4