Run Trigger Page: Configurable number of recent configs

[AchimGaedkeLynker]

Some users are heavily relying on running DAGs manually by altering or repeating previous configurations.
The number (more precisely limit) of previous runs sent to the trigger.html selector is 5 and quite low for this use-case.

This PR makes the number of recent DAG run configurations configurable.

Questions:

• Are there any unit tests required here?
• I (speaking for the in house users) would love that feature sooner than later, I've put version 2.9.0 into the configuration YML, I believe that's the earliest when sticking to semantic versioning... and the change is liked by the airflow community...

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[dirrao]

It would be great if you can add the test cases for this new change. If existing test cases are not available due to feasibility, then I would like to hear from others.
Can you attach the screenshot for the affected form/page?

[AchimGaedkeLynker]

I'd be really happy getting advice on unit tests, figuring out the test/CI setup for each project is always daunting.

This PR seems to have a decent example for unit-testing configurations. - @dirrao , what's your take?
https://github.com/apache/airflow/pull/35460/files#diff-603905684cc5476d24f0294f961251bbb9852f881803c52bedaeb220a82db11d

[AchimGaedkeLynker]

Screenshot as requested - it is the number of entries in the "Select Recent Configurations" dropdown

[jscheffl]

Without major rework of the overall code in the views it is hard to add tests - also other options with parameters are not explicitly tested. Also other parameters like show_trigger_form_if_no_params are not tested.

I assume it is good like this.

As it is a new feature and not a bug I'd not merge it into a patch release - but 2.9.0 is good as well.

THANKS for the contribution!