Bump net-imap from 0.5.4 to 0.5.6

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.5.4 to 0.5.6.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](https://github.com/ruby/net-imap/compare/v0.5.4...v0.5.6)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -372,7 +372,7 @@ GEM
     mutex_m (0.2.0)
     net-http (0.6.0)
       uri
-    net-imap (0.5.4)
+    net-imap (0.5.6)
       date
       net-protocol
     net-pop (0.1.2)