Pulsar: remove deprecated trigger name

[kunwooy]

This commit removes the deprecated trigger name msgBacklog in Pulsar scaler. msgBacklogThreshold should be used instead.

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Changelog has been updated and is aligned with our changelog requirements
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #6059
(The original issue was intended to introduce a scaler to non-persistent Pulsar topics. But after some research, I found out that the proposed metric is infeasible)

Relates to #

[JorTurFer]

Could you sign the branch please?

To add your Signed-off-by line to every commit in this branch:
1.Ensure you have a local copy of your branch by checking out the pull request locally via command line.
2. In your local branch, run: git rebase HEAD~2 --signoff
3. Force push your changes to overwrite the branch: git push --force-with-lease origin main

[kunwooy]

@JorTurFer Done!

[JorTurFer]

There are failing some tests:

and there is also an static check failing:

Could you take a look please? 🙏

[kunwooy]

@JorTurFer Done! (for real this time)

[JorTurFer]

/run-e2e pulsar
Update: You can check the progress here