Rename catboost

[julia-shenshina]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Closing issues

closes #737

[github-actions]

🚀 Deployed on https://deploy-preview-779--etna-docs.netlify.app

[codecov-commenter]

Codecov Report

Merging #779 (8266d43) into master (01b349d) will decrease coverage by 34.16%.
The diff coverage is 42.85%.

@@             Coverage Diff             @@
##           master     #779       +/-   ##
===========================================
- Coverage   83.74%   49.57%   -34.17%     
===========================================
  Files         123      123               
  Lines        6870     6895       +25     
===========================================
- Hits         5753     3418     -2335     
- Misses       1117     3477     +2360     

Impacted Files	Coverage Δ
etna/models/catboost.py	64.28% <38.46%> (-35.72%)	⬇️
etna/models/__init__.py	100.00% <100.00%> (ø)
etna/commands/__init__.py	0.00% <0.00%> (-100.00%)	⬇️
etna/commands/backtest_command.py	0.00% <0.00%> (-97.06%)	⬇️
etna/commands/forecast_command.py	0.00% <0.00%> (-94.88%)	⬇️
etna/commands/__main__.py	0.00% <0.00%> (-87.50%)	⬇️
etna/commands/resolvers.py	0.00% <0.00%> (-80.00%)	⬇️
etna/analysis/outliers/density_outliers.py	22.44% <0.00%> (-75.52%)	⬇️
etna/datasets/datasets_generation.py	27.02% <0.00%> (-72.98%)	⬇️
etna/transforms/timestamp/time_flags.py	27.02% <0.00%> (-72.98%)	⬇️
... and 74 more

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[martins0n]

You should add record to CHANGELOG.md