Upgrade to RegistryTools v2, and require at least RegistryTools 2.1.0

[DilumAluthge]

Reverts #1268 (which reverted #1267)

[giordano]

I mean, I reverted #1267 because it was broken. Unless you fix the breaking changes, this PR is not going to be accepted in the present form.

To make it crystal clear, we don't test registration at all, so don't expect to be able to catch errors that way. Of course, unless you volunteer to set up a mock registry for testing, which would be much welcome).

[DilumAluthge]

d9f2fbf fixes the breaking changes. AFAICT that's the only place where RegistryTools is used.

Yeah, I don't know how to write a test for this, short of setting up a test registry, etc, like we do for RegistryCI.

[DilumAluthge]

Bump @giordano @staticfloat

The second commit here should fix the breaking changes. Obviously I have no way of testing that. The commit is based on my reading of the RegistryTools source code, as well as the comment in the RegistryTools README about upgrading to the latest RegistryTools release.

[staticfloat]

We tested this live on the #ci-dev call, and it successfully opened a PR to General: JuliaRegistries/General#80444

I believe this should be good to go.

[staticfloat]

Tested on Julia v1.7