Serve Trix from the action_text-trix gem instead of a vendored copy

The importmap pinned a hand-vendored copy of Trix 2.1.19
(vendor/javascript/trix.esm.min.js) that dependency tooling can't see.
The action_text-trix gem already in the bundle ships the same 2.1.19
release as app/assets/javascripts/trix.js and adds it to the asset
precompile list, so pin against that instead and drop the vendored blob.
Trix updates now arrive through ordinary gem bumps, visible to
Dependabot and security advisories.

The gem's build is UMD rather than ESM, which makes no difference here:
application.js imports "trix" only for its side effect of registering
the trix-editor and trix-toolbar custom elements, and the UMD build does
that via its globalThis branch. No bindings are imported from the
module, and Trix styling remains app-owned in
app/assets/stylesheets/actiontext.css.

config/importmap.rb

@@ -5,7 +5,7 @@ pin "@hotwired/stimulus-loading", to: "stimulus-loading.js"
 pin "@hotwired/turbo-rails", to: "turbo.js"
 pin "@rails/actioncable", to: "actioncable.esm.js"
 pin "@rails/request.js", to: "@rails--request.js" # @0.0.8
-pin "trix", to: "trix.esm.min.js" # @2.1.19
+pin "trix", to: "trix.js"
 pin "@rails/actiontext", to: "actiontext.js"
 pin "highlight.js", to: "highlight.js/core.js"