Add Ruby 3.1 support to 2.7 branch

[stomar]

Please consider backporting d9d8dcc6bae1774a033ad488cea6217db3a3ebf3 to the 2.7 branch.

Currently, using the latest stable release (Mail 2.7.1) with Ruby 3.1 makes it necessary for end users to add net-smtp to their Gemfile.

[johnnyshields]

@mikel this would be a big help to many users if you could do this.

[mikel]

This has been discussed at length in https://github.com/mikel/mail/pull/1439 and is not a simple fix unfortunately.