[master] FOSS UpdateManager + Remove LegalChecker

[turbolay]

This PR is a port of all the PRs that have been made on the Downsize+FOSS branch about LegalChecker and UpdateManager

#12980
#13027
#13050
#13098
#13085
#13100

There is only one tweak: #12980 breaks backward compatibility as the backend doesn't return a LegalDocument which breaks at least the UpdateManager. So I reverted changes backend side.

I didn't have time to test on Regtest so maybe I forgot some stuff, would be great to have some reviews/tests

[lontivero]

What's the purpose of this PR?

[turbolay]

What's the purpose of this PR?

As we mentioned last week, it is better to merge the UpdateManager to master right now so the client keeps discovering releases if the backend is not available.

Considering that the new UpdateManager is built on one of your commit to remove LegalDocuments, and that the coinjoin service is stopped, I also included the removal of legal documents.

[lontivero]

I see, it makes sense. thank you

[yahiheb]

Tested this a bit, I don't get the Legal Documents, so this partially fixes #13096.

[turbolay]

I'm testing this extensively today and will merge tonight if no problem arise

[turbolay]

tACK, this PR is a big improvement in many ways, as it fixes many problems on production or that have been introduced in Downsize+FOSS by #13027

Also thanks to this PR the auto-updater will work on 2.0.8 when we will transition to the Downsize+FOSS branch. On the other hand, it will be broken on all prior versions.