fix: Link to intermediate changelog

[adamdmharvey]

Whenever the Backstage service user updates the Version Updates PR, the description has a message which is unusable.

I've tried clicking it before, as have colleagues of mine, just to be redirected to a 404. That's because the message links to an intermediate changelog which doesn't yet actually exist, and the message link is hardcoded to the master branch.

This updates the action to make this link use the intermediate branch name so that users can preview the version updates in the pending changelog before the actual release is cut.

The link in the description codes to:
https://github.com/backstage/backstage/blob/master/docs/releases/v1.15.0-next.1-changelog.md

While it SHOULD be:

https://github.com/backstage/backstage/blob/changeset-release/master/docs/releases/v1.15.0-next.1-changelog.md
(which does work and let's me preview the next changelog)

[adamdmharvey]

FYI @Rugvip @freben @benjdlambert there's no CODEOWNERS in this repo and I'm not able to tag the maintainers team for some reason. (may not directly have permissions to this repo?)

[adamdmharvey]

I guess the only question in my mind is before merge/after merge mechanics, in that if I look at this older PR:

backstage/backstage#17831

Then the link NOW works, and with my change I have a feeling it wouldn't? Unless this action rewrites it just before merge, which it may not.

So maybe this is an experiment and welcome pros/cons/suggestions!

Potentially we could even add BOTH links, one to the intermediate branch and one to master and sort of parenthetically note which one might work when. sort of covers both use cases - bit brute force but maybe more user friendly?

[benjdlambert]

@adamdmharvey another suggestion would be to use the commit SHA instead of the latest commit, rather than using the branch name which would always be changeset-release/master.

[benjdlambert]

@adamdmharvey another suggestion would be to use the commit SHA instead of the latest commit, rather than using the branch name which would always be changeset-release/master.