Update dependency net-imap to v0.6.4.1

Author: renovate[bot]

resources/ext/build-scripts/jruby-stdlib-gem-list.txt

@@ -1,7 +1,7 @@
 matrix 0.4.3
 minitest 5.26.2
 net-ftp 0.3.9
-net-imap 0.5.12
+net-imap 0.6.4.1
 net-pop 0.1.2
 net-smtp 0.5.1
 power_assert 2.0.5