CI: pin ubuntu-latest jobs to ubuntu-26.04 (#5983)

GitHub is moving the ubuntu-latest label from Ubuntu 24.04 to 26.04
between October 19 and November 19, 2026 (actions/runner-images#14748).
Pin every ubuntu-latest job to ubuntu-26.04 so the switch does not
change the toolchain under our jobs, and so the behaviour does not
depend on when GitHub reaches our runs.

Jobs pinned to ubuntu-24.04 and ubuntu-24.04-arm are unchanged.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/workflows/dependency-review.yml

@@ -18,7 +18,7 @@ permissions:
 
 jobs:
   dependency-review:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     steps:
       - name: Harden Runner
         uses: step-security/harden-runner@dc2769abcc0fcfbadfc1df54081c1608579ef6b2 # v2.22.2