docs: fix outdated Maven subtitle in INSTALL.md

[he1l0world]

Description

This PR updates the Maven setup subtitle in INSTALL.md to reflect the correct version (3.9.9), aligning it with the updated download URL introduced in PR #10800.

In that PR, the download link for Maven was correctly updated from 3.6.0 to 3.9.9, but the subtitle above the instructions still referenced the older version (3.6.0). This follow-up change ensures consistency and clarity.

Types of changes

• Breaking change (fix or feature that would cause existing functionality to change)
• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Enhancement (improves an existing feature and functionality)
• Cleanup (Code refactoring and cleanup, that may add test cases)
• build/CI
• test (unit or integration test code)

Feature/Enhancement Scale or Bug Severity

Feature/Enhancement Scale

• Major
• Minor

Bug Severity

• BLOCKER
• Critical
• Major
• Minor
• Trivial

Screenshots (if appropriate):

How Has This Been Tested?

Tested by reviewing the Markdown.

[boring-cyborg]

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Here are some useful points:

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[he1l0world]

Hi team,

I’m a new joiner to the community and am currently building CloudStack locally by following the instructions in INSTALL.md.

While going through the setup, I noticed this minor inconsistency and thought it’d be a good opportunity to make my first contribution.

I'm looking forward to contributing more and learning from the project and the community!

Thank you!

[DaanHoogland]

welcome @he1l0world , hope you enjoy it. thanks for the doc-update

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 16.60%. Comparing base (2d669db) to head (f8d44f5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #11012      +/-   ##
============================================
- Coverage     16.60%   16.60%   -0.01%     
  Complexity    13924    13924              
============================================
  Files          5730     5730              
  Lines        508082   508082              
  Branches      61770    61770              
============================================
- Hits          84388    84386       -2     
- Misses       414259   414260       +1     
- Partials       9435     9436       +1     

Flag	Coverage Δ
uitests	3.93% <ø> (ø)
unittests	17.50% <ø> (-0.01%)	⬇️

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[he1l0world]

welcome @he1l0world , hope you enjoy it. thanks for the doc-update

Thank you! @DaanHoogland

[he1l0world]

Thanks for the review and approval! Since I don't have the merge permissions, kindly help with the merge when you get a chance. Thank you! @DaanHoogland

[boring-cyborg]

Awesome work, congrats on your first merged pull request!