fix: LEAP-1887: Add project color validation

[triklozoid]

• validation added to ProjectSerializer
• test will be in LSE part to cover both repos

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[codecov]

Codecov Report

Attention: Patch coverage is 12.50000% with 7 lines in your changes missing coverage. Please review.

Project coverage is 77.25%. Comparing base (6c6b149) to head (4854da1).
Report is 4 commits behind head on develop.

Files with missing lines	Patch %	Lines
label_studio/projects/serializers.py	12.50%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7349      +/-   ##
===========================================
- Coverage    77.29%   77.25%   -0.04%     
===========================================
  Files          188      188              
  Lines        14618    14626       +8     
===========================================
+ Hits         11299    11300       +1     
- Misses        3319     3326       +7     

Flag	Coverage Δ
pytests	77.25% <12.50%> (-0.04%)	⬇️

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[triklozoid]

/git merge

Workflow run
Successfully merged: 384 files changed, 56 insertions(+), 4785 deletions(-)