fix(deps): patch quinn-proto to 0.11.15

[tobyhede]

Summary

• Bumps quinn-proto (transitive dependency via quinn) from 0.11.14 to 0.11.15
• Fixes advisory GHSA-4w2j-m93h-cj5j (High severity), flagged by Dependabot/Vanta
• A prior fix already bumped quinn-proto to >=0.11.14 for an earlier advisory; this bumps one patch version further
• cargo update -p quinn-proto --precise 0.11.15 resolved cleanly within the existing quinn = 0.11.8 constraint, so no quinn version bump or Cargo.toml change was needed

Test plan

• cargo build --workspace succeeds
• cargo test --lib -p cipherstash-proxy (132 tests) passes single-threaded
• Full integration suite (mise run test, requires docker-compose Postgres) not run locally — no docker available in this environment; CI will cover it

[freshtonic]

Approving.

Verified against the advisory rather than the PR body:

• GHSA-4w2j-m93h-cj5j — "Remote memory exhaustion in quinn-proto from unbounded out-of-order stream reassembly", High, quinn-proto >= 0.1.0, < 0.11.15 → first patched 0.11.15. The bump lands exactly on the floor.
• Diff is a single Cargo.lock entry (version + checksum), no Cargo.toml change — consistent with the claim that quinn = 0.11.8's existing constraint already admits 0.11.15, so there's no semver-incompatible surface here.
• No other quinn-proto entry left in the lockfile.

CI is fully green, including all four Postgres integration matrices (14/15/16/17) and the performance regression check — so the one unchecked box in the test plan (integration suite not runnable locally without docker) is covered.

Nothing further from me.