Remove RenderEngineManager's inheritance from Singleton base class

[iche033]

🦟 Bug fix

Summary

Related PR: #734.

Implements the ABI breaking change as mentioned in the todo note added in #734.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #874 (916730a) into main (9a76f1f) will not change coverage.
Report is 1 commits behind head on main.
The diff coverage is 100.00%.

❗ Current head 916730a differs from pull request most recent head 4619316. Consider uploading reports for the commit 4619316 to get more accurate results

@@           Coverage Diff           @@
##             main     #874   +/-   ##
=======================================
  Coverage   75.10%   75.10%           
=======================================
  Files         176      176           
  Lines       16853    16853           
=======================================
  Hits        12658    12658           
  Misses       4195     4195           

Files Changed	Coverage Δ
src/RenderEngineManager.cc	76.89% <100.00%> (ø)