Make Towncrier compatible with the release script

[manuroe]

Final part of element-hq/element-ios#4393

The release script needs to add an entry to CHANGES.md to indicate dependency updates like at https://github.com/vector-im/element-ios/blame/v1.4.9/CHANGES.rst#L68.
We want to use towncrier but we have no issue number for doing releases. We could get the release PR number but this is complex to implement. So we have no number that will allow towncrier to build and add a link to a release change.

With this template update, the new filename rule (x-nolink-) allows to add an entry to CHANGES.md without any link automatically built by towncrier.

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The last commit is for having a valid MD link for PR changelog entry like:

🧱 Build

• CHANGES.md: Use towncrier to manage the change log. More info in CONTRIBUTING. (#1196, #4393)

Source:

🧱 Build

- CHANGES.md: Use towncrier to manage the change log. More info in [CONTRIBUTING](CONTRIBUTING.md#changelog). ([#1196](https://github.com/matrix-org/matrix-ios-sdk/pull/1196), [#4393](https://github.com/vector-im/element-ios/issues/4393))