Upgrade Corepack to 0.34.7 (#99959)
Needs a version bump for pnpm 12. We obviously can't apply the pnpm
lockfile to the corepack install, but pinning the version here should be
sufficient as corepack is zero-dependency (and makes a big deal about
that). We're using 0.34.7 because newer versions drop Node 20 support.
