ENH: Add age of PR to message

[khalford]

Description:

Change the PR staleness message to have the number of days the PR has been open for.

Submitter:

Have you done the following?:

• Labeled the pull request from the following? major | minor | patch or documentation | workflow
• Updated the documentation?
• Added unit tests for new / untested code?
• Deployed the changes onto dev-chatops.nubes.rl.ac.uk?

New / Existing features:

• Updated the version.txt and docker-compose.yml files?

---

Reviewer:

Have you checked the following?:

• Version change is appropriate to the code changes? Semantic versioning documentation here
• Unit test code coverage is acceptable?
• Do the CI jobs pass?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (c256729) to head (81d2dca).
⚠️ Report is 15 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #136      +/-   ##
==========================================
+ Coverage   98.39%   99.79%   +1.40%     
==========================================
  Files          36       20      -16     
  Lines        2182      984    -1198     
==========================================
- Hits         2147      982    -1165     
+ Misses         35        2      -33     

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