Chore: Improve PR title validation regex

[sampaiodiego]

Proposed changes (including videos or screenshots)

I don't know how it happened, but looks like the regex was accepting some invalid titles like for example:

[IMPROVE][EE] Description

In this case, it should be invalid because the seconds group ([EE]) is invalid, but since [IMPROVE] is valid, the regex has a match and so the title is considered valid.

The new regex will validate the second group as well, if present, and allow only the values [ENTERPRISE] and [APPS].

I've used this list of valid and invalid titles to check the regex:

[NEW] ok
[BREAK] ok
[IMPROVE] ok
[FIX] ok
[BREAK][ENTERPRISE] ok
[BREAK][APPS] ok
Regression: ok
Chore: ok
Revert: ok
i18n: ok
Bump ok
Release 1.2.3
Merge master into develop

[NEW]not ok
[NEW][NOPE] not ok
Regression:not ok
Bump: not ok
Release version 1.2.3 not ok

I'm considering having somes tests to make sure the regex works as expected and having a reference to check at some point. wdyt?

Issue(s)

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