FIX: proper details / summary excerpt (#30229)

It doesn't make much sense to have the content of a `<details>` in an excerpt so I replaced them with "▶ summary" instead.

That way, they can't be (ab)used in user cards for example.

Reference - https://meta.discourse.org/t/335094

spec/lib/pretty_text_spec.rb

@@ -923,16 +923,10 @@ RSpec.describe PrettyText do
         ).to eq("![car](http://cnn.com/a.gif)")
       end
 
-      it "should keep details if too long" do
+      it "replaces details / summary with the summary" do
         expect(
           PrettyText.excerpt("<details><summary>expand</summary><p>hello</p></details>", 6),
-        ).to match_html "<details class='disabled'><summary>expand</summary></details>"
-      end
-
-      it "doesn't disable details if short enough" do
-        expect(
-          PrettyText.excerpt("<details><summary>expand</summary><p>hello</p></details>", 60),
-        ).to match_html "<details><summary>expand</summary>hello</details>"
+        ).to match_html "▶ expand"
       end
 
       it "should remove meta information" do