.github/workflows: update actions to Ubuntu 24.04

Ubuntu 24.04 no longer packages SageMath, as noted on
https://ask.sagemath.org/question/77308/installing-sagemath-in-ubuntu-2404/
The only solution for now seems to build from source, according to
https://sagemanifolds.obspm.fr/install_ubuntu.html

Moreover, Ubuntu 24.04 no longer provides Python 3.7, according to
https://github.com/actions/python-versions/blob/3.13.1-12154081405/versions-manifest.json

And install pandoc explicitly as it is no longer installed by default.

.github/workflows/linux.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   linux-test:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
         compiler:
@@ -19,7 +19,6 @@ jobs:
           - "3.10"
           - "3.9"
           - "3.8"
-          - "3.7"
           - "pypy-3.10"
           - "pypy-3.9"
           - "pypy-3.8"
@@ -51,8 +50,8 @@ jobs:
             libsdl2-dev \
             libz3-dev \
             openjdk-11-jdk \
+            pandoc \
             pkgconf \
-            sagemath \
             texlive-fonts-extra \
             texlive-lang-french \
             texlive-science

.github/workflows/mingw_linux.yml

@@ -4,7 +4,7 @@ on: [push, pull_request]
 
 jobs:
   mingw-linux-test:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     strategy:
       matrix:
         compiler: