Change USERNAME env to KUBEFLOW_USERNAME

[lucferbux]

Description

This pull request includes changes to update the environment variable for the username from USERNAME to KUBEFLOW_USERNAME across multiple files in the frontend client. The most important changes include updates to environment variables, documentation, and hooks.

How Has This Been Tested?

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[Al-Pragliola]

Everything seems okay -- Can I only kindly ask to also change the env @ devenv/configs/tiltfiles/frontend.tiltfile at line 85 "USERNAME" => "KUBEFLOW_USERNAME"

[lucferbux]

Everything seems okay -- Can I only kindly ask to also change the env @ devenv/configs/tiltfiles/frontend.tiltfile at line 85 "USERNAME" => "KUBEFLOW_USERNAME"

Ah perfect, I saw that but wasn't sure if it was related, but now I see it's for frontend, thanks a lot @Al-Pragliola

[Al-Pragliola]

/lgtm

[rareddy]

/lgtm
/approve

[google-oss-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Al-Pragliola, rareddy

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