Remove Associate role from on-call rotation

Why is this change being made?

With a new Associate Engineer role introduced earlier this year, I think a change to on-call eligibility should follow, which takes this role into account. This is an entry-level role and I think it shouldn't be included in full on-call rotation, especially without any targeted onboarding to prepare for such duty. As a quick solution, I suggest excluding Associate engineers from on-call rotation, as there have already been some shift assignments that will need reverting if this change gets merged.

Additional suggestions / iterations:

• Allow Associates to shadow on-call engineers and learn from them (do we need a process for this or it would be more on a "opt-in" basis and up to Associate to decide?)
• Prepare an onboarding that would introduce Associates to on-call

If this gets approved, I'll open MR that changes the eligible list export to skip Associate role.

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