[Backport support/2.14] GHA: add Ubuntu 25.10

[backbot-ci]

Backport of #10607 to support/2.14, triggered by a label.

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This is an automated backport PR. Please review it carefully before merging.

[yhabteab]

Hm, sorry guys! I don't know why I didn't notice this yesterday but ubuntu 25.10 ships Boost 1.88 and I either have to revert this or backport this PR to the 2.14 support branch as well:

• Support Boost 1.88 on Windows #10419

/cc @julianbrost

[yhabteab]

Sorry again, I've referenced the wrong PR. This is the correct PR that has to be backported:

• Bump Boost shipped for Windows to v1.87 #10278

[julianbrost]

I don't know why I didn't notice this yesterday

Probably because you enabled auto-merge. If the PR adds new checks and these aren't marked as required yet, that will happily merge the PR even if the new checks failed (because all required checks passed and that's all that auto-merge cares about). Thus, it's always a good idea to check that all outstanding checks are shown as required before enabling auto-merge.

I either have to revert this or backport this PR to the 2.14 support branch as well

Reverting would mean we would then have to also exclude Ubuntu 25.10 (and most likely all newer Ubuntu versions) from package builds of future 2.14.x releases. That's both annoying for us (doing that wasn't really planned in out package tooling) and possibly for users (if their master is on Icinga 2.14.x, they couldn't use newer Ubuntu versions on any agents without violating our version compatibility recommendations). Hence, I'd say it's best to make sure that active support/* branches work with all our supported distribution versions.

[yhabteab]

Thus, it's always a good idea to check that all outstanding checks are shown as required before enabling auto-merge.

Oh yeah, that's my fault.

Hence, I'd say it's best to make sure that active support/* branches work with all our supported distribution versions.

Ok. I'll let the bot backport it then.