Use link refs for PR links in changelog

[straight-shoota]

This makes the individual entries easier to read with just the PR number and the URL is detached at the end of the current section.

Example diff that this change produces in CHANGELOG.md:

-- Allow multiple parameters and blocks for operators ending in `=` ([#14159](https://github.com/crystal-lang/crystal/pull/14159), thanks @HertzDevil)
+- Allow multiple parameters and blocks for operators ending in `=` ([#14159], thanks @HertzDevil)
+
+[#14159]: https://github.com/crystal-lang/crystal/pull/14159