Update net-imap in lobsters

[eregon]

• Version 0.5.11 has a hard reference to Ractor: uninitialized constant Net::IMAP::Config::AttrTypeCoercion::Ractor with release 0.5.11 net-imap#527

[eregon]

With this truffleruby-head is able to run lobsters.
I can't remove the exclude in

ruby-bench/.github/workflows/test.yml

Lines 51 to 54 in bdc7fa9

	# lobsters: commonmarker fails to install
	# ruby-lsp: rbs_extension.so: undefined symbol: ruby_vm_at_exit
	# shipit: cannot load such file -- pty
	extra_args: "--excludes=knucleotide,lobsters,ruby-lsp,shipit"

because that uses the latest release, but I'll remove it in https://github.com/truffleruby/truffleruby/blob/4180ab960fe223d333e86b6f3263eb1e29d8f942/.github/workflows/ruby-bench.yml#L26

BTW the previous commonmarker error is solved by #465 :)