fix(slack): truncate slack body message if limit is reached

[pfongkye]

In an attempt to fix this issue:

• Truncate text that exceeds 3000 characters as Slack rejects those blocks with invalid_blocks errors. There was a bug in the truncation which did not take into account the appended message length properly. The latter resulted in exceeding the Slack limit in some cases.
• Adjust body-parser limit to default 5mb which can be configured using CONFIG.requestBodySizeLimit. This is due to the fact that Gitlab webhook payloads routinely exceed the previous 500kb when MRs carry long descriptions.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.13%. Comparing base (f4e10c4) to head (37daf65).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #171      +/-   ##
==========================================
+ Coverage   89.10%   89.13%   +0.02%     
==========================================
  Files         103      103              
  Lines        2248     2254       +6     
  Branches      429      431       +2     
==========================================
+ Hits         2003     2009       +6     
  Misses        232      232              
  Partials       13       13              

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