Fixed CI workflows.

- Since recently, an important part of our CI workflows fails.
  - Workflows "Build DAPHNE" (main.yml) and "Build (& Deploy) Docs" (docs.yml).
  - See commits 717fb4b and 457d127.
- The reason is that these workflows are configured to run on ubuntu-20.04, which has recently become unsupported by GitHub.
  - See actions/runner-images#11101.
- This commit fixes the problem by configuring the workflows to run on ubuntu-latest (just like the third, still working, workflow "check-clang-format" (check-clang-format.yml)).
  - ubuntu-latest currently means ubuntu-24.04.
  - For the "Build DAPHNE" workflow, the host OS should not be that crucial, since we anyway start our own container (daphneeu/github-action:latest) from the host system.
  - For the "Build (& Deploy) Docs" workflow, a newer version of Ubuntu should work fine (just a few general commands like sed and a few 3rd-party actions, which either don't specify any requirements regarding the host OS or specify to support ubuntu-24.04 and ubuntu-lastest).
- This fix has been tested on a fork first.

Author: pdamme

.github/workflows/docs.yml

@@ -10,7 +10,7 @@ on:
 jobs:
   build:
     name: Build Docs
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     steps:
       - name: Checkout
         uses: actions/checkout@v4

.github/workflows/main.yml

@@ -10,7 +10,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-latest
     container: daphneeu/github-action:latest
     env:
       # fix usage of tput in build script: