Bump Markdown timeout from 3 to 6 seconds in test

We've been seeing increased flaky failures due to the timeout
exceeding 3 seconds. With the additional regexp in
e38e2adc, we see that performance has
gotten worse:

```
1.376477   0.005843   1.382320 (  1.423449)
```

Previously it was:

```
0.566795 0.000933 0.567728 ( 0.569076)
```

Closes https://gitlab.com/gitlab-org/gitlab/issues/35269

spec/support/shared_examples/models/concern/issuable_shared_examples.rb

@@ -4,7 +4,7 @@ shared_examples_for 'matches_cross_reference_regex? fails fast' do
   it 'fails fast for long strings' do
     # took well under 1 second in CI https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/3267#note_172823
     expect do
-      Timeout.timeout(3.seconds) { mentionable.matches_cross_reference_regex? }
+      Timeout.timeout(6.seconds) { mentionable.matches_cross_reference_regex? }
     end.not_to raise_error
   end
 end