New updater checklist

[LouisCAD]

• [x] Serialize the repositories from buildSrc and Settings to use them to get versions.
• [x] Automatically use the most stable version if a placeholder isn't resolved.
• [x] Update or remove the Gradle extension so there's no longer ignored configuration.
• [x] Add empty line after each version property
• [x] Make maven-metadata.xml files download parallel (coroutines + Dispatchers.IO?).
• [x] Make parsing of the files parallel (coroutines + Dispatchers.Default?).
• [x] Report hardcoded & dynamic versions (#126)
• [x] New versions sorting strategy #153
• [x] Show user-friendly error messages when unable to find available versions of an artifact from the given repositories.
• [x] Ease adding dependencies (#125)
• [x] Support cancellation for in-flight network requests.
• [x] Support update of the plugin itself
• [x] Support advertising Gradle update, link to migration guide and write the commands to update the wrapper (API here).
• [x] Keep user comments added above a property
• [x] Specify behavior for unused version entries (remove as before by default? add unused metadata comment?)
• [ ] Allow setting minimum stability level for version property (via a comment above it)
• [ ] Allow setting minimum version, via a comment above it (use-case: waiting for that bugfix release to upgrade)
• [ ] Allow overriding filtering & sorting mode per version property (via a comment above it)
• [ ] Allow disabling updates for a version property (via a comment above it)

[jmfayard]

Can be considered as done, we have now a way to configure the plugin via settings { }

[LouisCAD]

There's items left in the checklist, so the issue overall is still relevant and is part of the 1.0.0 milestone.

I need to keep it open to not miss the things that remain to be done.