chore(deps): bump quinn-proto to 0.11.15 (RUSTSEC-2026-0185) (#1269)

## Overview

Fixes high-severity advisory
[RUSTSEC-2026-0185](https://rustsec.org/advisories/RUSTSEC-2026-0185):
*Remote memory exhaustion in quinn-proto from unbounded out-of-order
stream reassembly*. Solution per the advisory is to upgrade to `>=
0.11.15`.

`quinn-proto` is pulled in transitively via `reqwest -> quinn ->
quinn-proto` across multiple workspace crates (`bottlecap`, `dogstatsd`,
`datadog-fips`, `libddwaf`, `datadog-agent-config`).

This blocks the v98 release — `cargo audit` is failing CI on `main`.

`cargo audit` after the bump shows zero vulnerabilities; the remaining
entries are pre-existing unmaintained-crate warnings (async-std,
buf_redux, multipart, rustls-pemfile, safemem, twoway).

## Testing

- `cargo audit` no longer reports a vulnerability.

Author: duncanista