2.2.10 is causing the dependency resolution on Rails 6-0-stable to fail:
```
Bundler could not find compatible versions for gem "railties":
In Gemfile-rails-6-0:
devise was resolved to 4.7.3, which depends on
railties (>= 4.1.0)
rails was resolved to 6.0.3.5, which depends on
railties (= 6.0.3.5)
responders (~> 3.0) was resolved to 3.0.1, which depends on
railties (>= 5.0)
Took 27.49 seconds
```
https://github.com/heartcombo/devise/runs/1905780158?check_suite_focus=true#step:5:23
The `railties` version 6.0.3.5 should work, given the other two are
using >= declarations, but it fails in 2.2.10.
Downgrading to 2.2.9 works.
And remove dupe entry in the exclude matrix.
In order to get Ruby 3 working we needed to install `rexml` as part of
the test dependencies, only done on the main Gemfile (Rails 6.1) and the
6.0 versions. (which are the only ones supported by Ruby 3.)
Devise itself doesn't require `rexml` as it does nothing with it, but a
dependency we use during tests seem to require it. I was able to track
it down to omniauth-openid -> rack-openid -> ruby-openid requiring it:
13a88ad644/lib/openid/yadis/xrds.rb (L1)
So while we have tests using omniauth-openid, we'll need this require in
place as well. Ideally that upstream version of ruby-openid should have
it, but it seems that one isn't updated in a while.
