fix(notifications): Convert slack notification title to rich text

[snigdhas]

Converting the title for slack notifications to use rich text instead of markdown so the issue title link works for iOS. This is a known issue with Slack's iOS app.

Visually, there should be no change here.

Fixes #82961

[codecov]

Codecov Report

Attention: Patch coverage is 97.63780% with 6 lines in your changes missing coverage. Please review.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...sentry/notifications/notifications/test_digests.py	66.66%	5 Missing ⚠️
...y/integrations/slack/message_builder/base/block.py	91.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #91637      +/-   ##
==========================================
+ Coverage   85.40%   87.92%   +2.51%     
==========================================
  Files       10178    10141      -37     
  Lines      583542   582448    -1094     
  Branches    22639    22445     -194     
==========================================
+ Hits       498366   512104   +13738     
+ Misses      84724    69930   -14794     
+ Partials      452      414      -38     

[cathteng]

lgtm on block kit builder