[rc2] Use public client auth for odsp e2e flows (#21091)

[Abe27342]

Description

Currently, we use the ropc flow with a confidential client to authenticate in our e2e/stress tests against odsp. This is less appropriate than using a public client flow, since our application registration really only needs to have delegated permissions. This PR adjusts things to use a public flow--I've updated the relevant app registrations to allow both forms of auth already.

Using a public flow also means our infrastructure setup here aligns exactly with this relatively recent MSFT guidance.

Cherry-pick of #21091. This change needs to be in every branch we plan on maintaining real service tests against odsp for, as I plan on removing the application configuration enabling confidential client auth.

[msfluid-bot]

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