Rename master_params to main_params

[awaelchli]

What does this PR do?

Tangential to #8774

We want to clean up our code of words and expressions that could have negative connotations.

Does your PR introduce any breaking changes? If yes, please list them.

None

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

[kaushikb11]

@awaelchli Still Draft?

[awaelchli]

nope!

[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #10105 (d52ceb1) into master (21a5867) will increase coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #10105    +/-   ##
========================================
+ Coverage      89%      93%    +4%     
========================================
  Files         182      182            
  Lines       16159    16162     +3     
========================================
+ Hits        14333    14952   +619     
+ Misses       1826     1210   -616