💥 Drop ruby 2.7 and 3.0 support, and require 3.1

[nevans]

Ruby 2.7 EOL was 2023-03-31. Ruby 3.0 EOL was 2024-04-23.

Currently, net-imap remains compatible with ruby 2.7. But some of my unmerged branches do use ruby 3.1 syntax (e.g: updated pattern matching and endless method definitions). It will be nice if I don't need to update those branches for compatibility with EOL rubies.

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NOTE: The next release after this is merged should be v0.5.0.

[nevans]

@shugo @hsbt Are you okay with bumping the minimum ruby version for the 0.5.x releases?

As a policy, I'd like to only change the minimum required ruby when we bump our major.minor version. And I'd like to only bump the major.minor version once a year, after the oldest version of ruby reaches its end of life. This way, net-imap's major.minor will be approximately synchronized with ruby's.