rename user container to main container

[EraKin575]

Fixes #15007

Proposed Changes

Renamed all UserContainer occurences to MainContainer in pkg package *

Release Note

[knative-prow[bot]]

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[knative-prow[bot]]

Welcome @EraKin575! It looks like this is your first PR to knative/serving 🎉

[knative-prow[bot]]

Hi @EraKin575. Thanks for your PR.

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[dprotaso]

/hold we shouldn't be duplciating work

https://github.com/knative/serving/issues/15007#issuecomment-2173124094

[EraKin575]

@dprotaso I think we can remove this from hold now

[dprotaso]

/ok-to-test

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.59%. Comparing base (af2c899) to head (5b1179f).

:exclamation: Current head 5b1179f differs from pull request most recent head 3fec7ce

Please upload reports for the commit 3fec7ce to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #15330 +/- ## ========================================== - Coverage 84.61% 84.59% -0.02% ========================================== Files 219 219 Lines 13584 13584 ========================================== - Hits 11494 11492 -2 - Misses 1724 1726 +2 Partials 366 366 ```

:umbrella: View full report in Codecov by Sentry.
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[ReToCode]

Also before doing the actual rename of constants here, I'd start with checking the first points in https://github.com/knative/serving/issues/15007. Can ALL the occurences (including the actual values) in serving, pkg and docs be changed without breaking any public API? Only if the answer is true, we should actually rename stuff.