fix(ci): update rubocop for ruby 4.0 and ignore brakeman false positive

[jaspermayone]

What

Fix the red lint and scan_ruby jobs in CI.

Why

• lint crashed with cannot load such file -- prism/translation/parser34. Rubocop 1.81.6 and rubocop-ast 1.47.1 do not work with Ruby 4.0.1.
• scan_ruby failed on one Brakeman warning (HTTP Verb Confusion). The warning is a false positive. The line above it already returns false for HEAD requests.

Changes

• Update rubocop from 1.81.6 to 1.92.0 and rubocop-ast from 1.47.1 to 1.50.0. Prism stays at 1.9.0.
• Add the Brakeman warning to config/brakeman.ignore with a note.
• Allow find_by_public_param, find_by_public_param! and find_by_public_id in Rails/DynamicFindBy. These are our own methods from PublicParamLookup. The autocorrect would break them.
• Fix the offenses that the crash had hidden. Layout and rescue fixes are autocorrected. Five RSpec/MessageSpies offenses in two specs now use expect(...).to receive.

Notes

PR #215 fails for the same reason. It needs a rebase after this PR merges.