[SECURITY] Bump net-imap from 0.4.19 to 0.4.20 (#973)

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.4.19 to 0.4.20.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](ruby/net-imap@v0.4.19...v0.4.20)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-version: 0.4.20
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

Gemfile.lock

@@ -231,7 +231,7 @@ GEM
     multipart-post (2.4.1)
     net-http (0.6.0)
       uri
-    net-imap (0.4.19)
+    net-imap (0.4.20)
       date
       net-protocol
     net-pop (0.1.2)