refactor: update MSRV to 1.84, enable MSRV-aware resolver

[jlizen]

📬 Issue #, if available:
fixes #1077

✍️ Description of changes:

Bumping our MSRV to 1.84 so that we have access to the MSRV-aware resolver. Also, enabling it at the workspace level.

I didn't go as far as enable 2024 edition, hence explicitly setting resolver = 3 to use the MSRV-aware one anyway.

We are starting to see CI failures due to our dependencies breaking MSRV, and rather than playing whack-a-mole with fixing it, let's do the minimal bump to sidestep this issue.

MSRV 1.8.4 was released about ~11 months ago, so this is fairly conservative (and anybody on older MSRV is already having a very bad time, probably).

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