spec(abci): fixes the spec to inform about the presence of invalid ex…

…tensions in `last_commit` (backport #2423) (#2525)

This is an automatic backport of pull request #2423 done by
[Mergify](https://mergify.com).
Cherry-pick of b537697 has failed:
```
On branch mergify/bp/v0.38.x/pr-2423
Your branch is up to date with 'origin/v0.38.x'.

You are currently cherry-picking commit b537697.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   spec/abci/abci++_comet_expected_behavior.md
	modified:   spec/abci/abci++_example_scenarios.md

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   spec/abci/abci++_app_requirements.md
	both modified:   spec/abci/abci++_basic_concepts.md
	both modified:   spec/abci/abci++_methods.md

```


To fix up this pull request, you can check it out locally. See
documentation:
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

---


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---------

Co-authored-by: lasaro <lasaro@informal.systems>