Merge pull request #10644 from Icinga/backport-10607-to-support/2.15

[Backport support/2.15] GHA: add Ubuntu 25.10

Author: yhabteab

.github/workflows/linux.yml

@@ -51,6 +51,7 @@ jobs:
           - ubuntu:22.04
           - ubuntu:24.04
           - ubuntu:25.04
+          - ubuntu:25.10
 
         platform:
           - linux/amd64