Bug: [CI] Add workflow to ensure a pull request contains at least one linked issue



### Description

Add a CI workflow to ensure that each pull request contains at least one linked issue. We wish to introduce this requirement for improved project tracking. The workflow should do nothing else: just succeed/fail the check based on the presence/absence of linked issues in a pull request, respectively.
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [ ] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [x] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR adds a new pull request CI check to ensure that all pull requests have at least one linked issue. 

However, GitHub does not send any webhooks when a pull request is linked with an issue, so the check does not get triggered automatically when an issue is linked for a pull request which is already open. One way to trigger it manually is to update either the pull request title or description/body, and another is to push commits. Since pushing new commits tends to trigger all other checks, editing the pull request description might be preferable in such scenarios.

In addition, the PR includes the following changes:

- Renames the `conventional-commit-check.yml` workflow file to `pr-convention-checks.yml`.
- Removes the workflow code that labelled pull requests whose titles didn't follow conventional commit standards, since we no longer use the label for any purpose nowadays. It was initially added as a means to filter pull requests by that label, when we introduced the check.

The pull request can be reviewed one commit at a time for better understanding.

## Motivation and Context
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Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

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Closes #3375.

## How did you test it?
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Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
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Temporarily modified the trigger to be `pull_request` instead of `pull_request_target` to test the workflow.

1. The check fails if the PR does not contain any linked issues.

   ![Screenshot of failed check due to absence of linked issues](https://github.com/juspay/hyperswitch/assets/22217505/a8f5a7ac-d96f-4512-ae6d-caf7ae1c0b5c)
   
   Link to check run: https://github.com/juspay/hyperswitch/actions/runs/7561263548/job/20589130650?pr=3376

2. The check succeeds if the PR contains at least one linked issue.

   ![Screenshot of successful check run](https://github.com/juspay/hyperswitch/assets/22217505/779214c3-4552-463d-8dbd-34f8a403c41f)
   
   Link to check run: https://github.com/juspay/hyperswitch/actions/runs/7561509816/job/20589902417?pr=3376

3. The check fails if any of the linked issues is closed.

   ![Screenshot of failed check run due to one of the issues being closed](https://github.com/juspay/hyperswitch/assets/22217505/2b4b1b44-8978-4cf4-96c6-54c23a223380)


## Checklist
<!-- Put an `x` in the boxes that apply -->

- [ ] I formatted the code `cargo +nightly fmt --all`
- [ ] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable
