Deprecating support for Ubuntu18

[himani2411]

Description of changes

• Deprecating support for Ubuntu18

Checklist

• Make sure you are pointing to the right branch and add a label in the PR title (i.e. 2.x vs 3.x)
• Check all commits' messages are clear, describing what and why vs how.
• Make sure to have added unit tests or integration tests to cover the new/modified code.
• Check if documentation is impacted by this change.

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[codecov]

Codecov Report

Merging #5436 (006467f) into develop (c157a81) will increase coverage by 0.01%.
The diff coverage is n/a.

❗ Current head 006467f differs from pull request most recent head b06b52c. Consider uploading reports for the commit b06b52c to get more accurate results

@@             Coverage Diff             @@
##           develop    #5436      +/-   ##
===========================================
+ Coverage    89.67%   89.68%   +0.01%     
===========================================
  Files          175      175              
  Lines        15014    15014              
===========================================
+ Hits         13464    13466       +2     
+ Misses        1550     1548       -2     

Flag	Coverage Δ
unittests	89.68% <ø> (+0.01%)	⬆️

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see 2 files with indirect coverage changes

[enrico-usai]

In future we should remove this byos (Bring your own scheduler) patch.

[himani2411]

Sure