📦 Drop digest from Gemfile (workaround for #576)

`digest` 3.2.1 was released since the last `net-imap` release.  And
apparently `bundle exec rake release` activates `digest` _before_
loading the Gemfile.  But, since `digest` is a default gem anyway, not
including it in the Gemfile should be okay, I think.

See the errors here:
* 1st attempt: https://github.com/ruby/net-imap/actions/runs/20212203897/job/58019727242
* Merged #575
* 2nd attempt: https://github.com/ruby/net-imap/actions/runs/20219369554/job/58038064382

Author: nevans

Gemfile

@@ -4,7 +4,7 @@ source "https://rubygems.org"
 
 gemspec
 
-gem "digest"
+# gem "digest"  # not included as a workaround for #576
 gem "strscan"
 gem "base64"