Drop `Proc#bind`

[Earlopain]

Proc responds to instance_exec since forever ago. Official Ruby docs only go back to 2.0, so I added that as the minimum version, if someone should for some reason still use rubies before that.

https://docs.ruby-lang.org/en/2.0.0/BasicObject.html#method-i-instance_exec

Stumbled over this because of https://github.com/rubocop/rubocop-performance/issues/464. The next version will be 3.0.0, seems appropriate to drop in a major version.