Bump version to 3.9.0

[jdeamicis]

References

• Bump version to 3.9.0 aws-parallelcluster#5769
• Bump version to 3.9.0 aws-parallelcluster-cookbook#2502

Checklist

• Make sure you are pointing to the right branch.
• If you're creating a patch for a branch other than develop add the branch name as prefix in the PR title (e.g. [release-3.6]).
• 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.

Please review the guidelines for contributing and Pull Request Instructions.

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[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (66f64ef) 90.03% compared to head (6ea7c98) 90.03%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #580   +/-   ##
========================================
  Coverage    90.03%   90.03%           
========================================
  Files           16       16           
  Lines         2699     2699           
========================================
  Hits          2430     2430           
  Misses         269      269           

Flag	Coverage Δ
unittests	90.03% <ø> (ø)

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