Update reference in README from 2.4.0 to 2.18.0

[cwperks]

Description

Small PR to update a stale reference to 2.4.0 in the README. The README is supposed to show open issues for the next unreleased 2.x version which is currently 2.18.0.

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.
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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.97%. Comparing base (0965d54) to head (d99cb0b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4806      +/-   ##
==========================================
+ Coverage   70.72%   70.97%   +0.25%     
==========================================
  Files         310      310              
  Lines       20938    20938              
  Branches     3326     3326              
==========================================
+ Hits        14808    14861      +53     
+ Misses       4384     4329      -55     
- Partials     1746     1748       +2     

see 9 files with indirect coverage changes