Remove all references to unsupported rewrite rules

[johnduffell]

I spent a while trying to get the documented rule to work, but the release versions of the rule were confusing and the rules hadn't been built by any recent version of scalafix and the older versions didn't work with the latest scala 2.13 versions. It looked like they were knocked up to get a specific migration done and then unmaintained.

I didn't have any luck trying to build them myself, then once I gave up I found a ready made built in scalafix rule that did exactly what I needed!

This PR updates the docs accordingly.

I also had a search through and found a few more outdated references to update.

[johnduffell]

updated from https://github.com/scalacenter/scala3-migrate/blob/ebb4a4087ed11899b9010f4c75eb365532694c0a/migrate/src/main/scala/migrate/utils/ScalafixService.scala#L75-L80

[som-snytt]

This LGTM but I'm not familiar with the scalafix ecosystem. It is weird to have links to unofficial code, in the context of migration tooling which I just saw advertised in a video.

The last link (the improvement) is possibly extraneous. "When in doubt, omit," as I just read in Tolstoy.

Probably it's worth squashing the commits for improved lines/commit.

I'll defer to @SethTisue to hit the green button.

[SethTisue]

@adpi2 is this something you would like to review, or do you know of an appropriate person?

if not, let's optimistically merge before too much longer

[som-snytt]

I'm able to merge pessimistically. ("It couldn't be any worse than...")

[johnduffell]

OK I've managed to merge and squash thanks to stackoverflow so it should be as clean as it ever will be!
I agree the changes themselves better even if not perfect.

[som-snytt]

It seems that the overview for old scala.reflect points to the current api URL newly taken over by Scala 3, if I skimmed correctly. Deciding now whether to visit Costco before closing, not sure if that is just config somewhere.

[adpi2]

Thanks! This is indeed a more honest view of the current state of our Scalafix rules.

[SethTisue]

Thank you @johnduffell! Very glad to have these changes. Sorry we were a bit pokey about responding.