Bump net-imap from 0.5.12 to 0.5.15

Bumps [net-imap](https://github.com/ruby/net-imap) from 0.5.12 to 0.5.15.
- [Release notes](https://github.com/ruby/net-imap/releases)
- [Commits](ruby/net-imap@v0.5.12...v0.5.15)

---
updated-dependencies:
- dependency-name: net-imap
  dependency-version: 0.5.15
  dependency-type: indirect
...

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

Author: dependabot[bot]

Gemfile.lock

@@ -98,7 +98,7 @@ GEM
     concurrent-ruby (1.3.5)
     connection_pool (3.0.2)
     crass (1.0.6)
-    date (3.4.1)
+    date (3.5.1)
     debug (1.11.0)
       irb (~> 1.10)
       reline (>= 0.3.8)
@@ -171,7 +171,7 @@ GEM
     mustermann-grape (1.1.0)
       mustermann (>= 1.0.0)
     mutex_m (0.3.0)
-    net-imap (0.5.12)
+    net-imap (0.5.15)
       date
       net-protocol
     net-pop (0.1.2)