feat(collections): stabilize index argument in iterable methods

[tomas-zijdemans]

This PR is a follow up to #6382

IMHO, adding the index argument should be quite natural and risk free.

I tried to keep the scope clean: Only stabilize what that PR added as unstable.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.80%. Comparing base (a496da2) to head (2e7ff9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7119      +/-   ##
==========================================
+ Coverage   94.61%   94.80%   +0.19%     
==========================================
  Files         634      618      -16     
  Lines       51801    51472     -329     
  Branches     9329     9276      -53     
==========================================
- Hits        49011    48798     -213     
+ Misses       2216     2122      -94     
+ Partials      574      552      -22     

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