[YUNIKORN-2992] Emit warning message in plugin mode

[pbacsko]

What is this PR for?

Emit a warning message about plugin mode removal.

What type of PR is it?

• - Bug Fix
• - Improvement
• - Feature
• - Documentation
• - Hot Fix
• - Refactoring

Todos

• - Task

What is the Jira issue?

https://issues.apache.org/jira/browse/YUNIKORN-2992

How should this be tested?

Screenshots (if appropriate)

Questions:

• - The licenses files need update.
• - There is breaking changes for older versions.
• - It needs documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.02%. Comparing base (31132ba) to head (a690773).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
pkg/plugin/scheduler_plugin.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #943      +/-   ##
==========================================
- Coverage   68.03%   68.02%   -0.01%     
==========================================
  Files          70       70              
  Lines        9194     9195       +1     
==========================================
  Hits         6255     6255              
- Misses       2732     2733       +1     
  Partials      207      207              

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[craigcondit]

NIT: yunikorn -> YuniKorn

[craigcondit]

+1 LGTM.