Onboard/offboard members & update process docs

[tallclair]

• Offboard @tallclair, @sfowl, @PushkarJ :clap:
• Onboard @ritazh & @SaranBalaji90 :tada:
• Remove references to the associate role
• Update process documentation based on https://docs.google.com/document/d/18spZOZMmi9_jjaZ1z4x_WO6ykrWH2mX1O2MV_42zvvk/edit#

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tallclair

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Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/kubernetes/committee-security-response/blob/main/OWNERS)~~ [tallclair] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[tallclair]

/hold

For final steering approval

[PushkarJ]

Thank you!

/lgtm

[tallclair]

/hold cancel

Confirmed with steering, we're good to move forward with the new process.