Update references to oauth2_proxy

[steakunderscore]

What this PR does / why we need it:

The custodian of the project has been shifted from bitly to pusher.
So this diff updates these references.

Which issue this PR fixes
None, didn't open an issue as it's a small docs change.

Special notes for your reviewer:

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[k8s-ci-robot]

Hi @steakunderscore. Thanks for your PR.

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[steakunderscore]

/assign @bowei

[aledbf]

@steakunderscore please don' assign PRs manually.

[aledbf]

@steakunderscore have you tested the new image with the instructions from the example?

[steakunderscore]

Hi @aledbf. Sorry for the delay. Just tested this and can confirm it works as expected.

[aledbf]

/lgtm

[aledbf]

@steakunderscore thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, steakunderscore

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[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@589c9a2). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4329   +/-   ##
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  Files             ?       88           
  Lines             ?     6569           
  Branches          ?        0           
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  Hits              ?     3853           
  Misses            ?     2286           
  Partials          ?      430

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