Add release notes for 2.18

[derek-ho]

Description

Adds release notes for 2.18

Issues Resolved

None
Is this a backport? If so, please add backport PR # and/or commits #, and remove backport-failed label from the original PR.
No

Do these changes introduce new permission(s) to be displayed in the static dropdown on the front-end? If so, please open a draft PR in the security dashboards plugin and link the draft PR here
No

Testing

None

Check List

• New functionality includes testing
• New functionality has been documented
• New Roles/Permissions have a corresponding security dashboards plugin PR
• API changes companion pull request created
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.66%. Comparing base (811f26d) to head (92302a8).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4834      +/-   ##
==========================================
- Coverage   70.90%   69.66%   -1.24%     
==========================================
  Files         310      320      +10     
  Lines       20950    21660     +710     
  Branches     3331     3456     +125     
==========================================
+ Hits        14855    15090     +235     
- Misses       4341     4783     +442     
- Partials     1754     1787      +33     

see 19 files with indirect coverage changes