Make changelog checks more robust

[AlexVelezLl]

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Product

KDS.

Desired behavior

Would be helpful to update our Check Changelog and Update Changelog workflows to make them more robust.

### Tasks
- [ ] Update the `Check PR for Changelog` workflow to check that every field of a changelog item exist, that its not empty and that they are different from the "default" values written in the [PR Template](https://github.com/learningequality/kolibri-design-system/blob/d3d40d2b2ec63e51f509b0eb5221a9fa2770edb3/.github/pull_request_template.md#L26).
- [ ] Update the `Check PR for Changelog` workflow to make sure that the `<!-- [DO NOT REMOVE-USED BY GH ACTION] CHANGELOG START -->` and `<!-- [DO NOT REMOVE-USED BY GH ACTION] CHANGELOG END -->` flags are present in the PR description.
- [ ] Find a way to separate multiple changelog items and have them render nicely in the PR description (tbd).
- [ ] Update the `Update Changelog` workflow to ignore any line that does not correspond to any of the changelog item fields.

Possible Tradeoffs

We will probably need to create script files to avoid having too much logic code written inside the gh workflow file.

[RONAK-AI647]

Hey @AlexVelezLl ,I wanna work on this issue .Will you give an opportunity.****

[AllanOXDi]

Hi @RONAK-AI647 I have assigned it to you- Thanks for contributing

[RONAK-AI647]

@AlexVelezLl @AllanOXDi Raised a PR ,kindly review .

[AlexVelezLl]

Thank you @RONAK-AI647! We will take a look

[MisRob]

Hi @RONAK-AI647, I wanted to mention that Learning Equality will be closed from December 23 to January 5.

[MisRob]

@RONAK-AI647 has made some good progress in https://github.com/learningequality/kolibri-design-system/pull/885, but we closed due to inactivity. To resolve this issue, the referenced PR can be used as basis + including review feedback from @rtibbles.