Suppress mail warning for "unused variable - disp_type_s"

e.g.:

```
zzak@mbp16 railties % bin/test test/application/assets_test.rb
Run options: --seed 32028

.............../Users/zzak/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mail-2.8.1/lib/mail/parsers/content_location_parser.rb:592: warning: assigned but unused variable - disp_type_s
I, [2023-10-15T10:38:33.005925 #97662]  INFO -- : [0ca46786-853f-4740-9e71-f644c4893502] Started GET "/assets/demo.js" for 127.0.0.1 at 2023-10-15 10:38:33 +0900
E, [2023-10-15T10:38:33.006641 #97662] ERROR -- : [0ca46786-853f-4740-9e71-f644c4893502]
[0ca46786-853f-4740-9e71-f644c4893502] ActionController::RoutingError (No route matches [GET] "/assets/demo.js"):
[0ca46786-853f-4740-9e71-f644c4893502]
../Users/zzak/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mail-2.8.1/lib/mail/parsers/content_location_parser.rb:592: warning: assigned but unused variable - disp_type_s
..../Users/zzak/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mail-2.8.1/lib/mail/parsers/content_location_parser.rb:592: warning: assigned but unused variable - disp_type_s
I, [2023-10-15T10:38:33.973335 #97660]  INFO -- : [fa848d82-0d80-4b23-925b-73fa5d0f47d4] Started GET "/posts?debug_assets=true" for 127.0.0.1 at 2023-10-15 10:38:33 +0900
I, [2023-10-15T10:38:33.975021 #97660]  INFO -- : [fa848d82-0d80-4b23-925b-73fa5d0f47d4] Processing by PostsController#index as HTML
I, [2023-10-15T10:38:33.975209 #97660]  INFO -- : [fa848d82-0d80-4b23-925b-73fa5d0f47d4]   Parameters: {"debug_assets"=>"true"}
I, [2023-10-15T10:38:33.977560 #97660]  INFO -- : [fa848d82-0d80-4b23-925b-73fa5d0f47d4] Completed 200 OK in 2ms (Views: 1.9ms | ActiveRecord: 0.0ms | Allocations: 1266)
......./Users/zzak/.rbenv/versions/3.2.2/lib/ruby/gems/3.2.0/gems/mail-2.8.1/lib/mail/parsers/content_location_parser.rb:592: warning: assigned but unused variable - disp_type_s
.

Finished in 4.842481s, 5.9887 runs/s, 23.3351 assertions/s.
29 runs, 113 assertions, 0 failures, 0 errors, 0 skips
```

For example:
https://buildkite.com/rails/rails/builds/94132#0186806f-8a23-4d07-ae8f-3b937f8517ae/1162-1171

See also rails#47484

Author: zzak

activesupport/lib/active_support/testing/strict_warnings.rb

@@ -17,6 +17,7 @@ module RaiseWarnings # :nodoc:
     SUPPRESSED_WARNINGS = Regexp.union(
       # TODO: remove if https://github.com/mikel/mail/pull/1557 or similar fix
       %r{/lib/mail/parsers/.*statement not reached},
+      %r{/lib/mail/parsers/.*assigned but unused variable - disp_type_s},
       %r{/lib/mail/parsers/.*assigned but unused variable - testEof}
     )