build(deps-dev): bump fastify from 2.13.1 to 2.15.1

Bumps [fastify](https://github.com/fastify/fastify) from 2.13.1 to 2.15.1.
- [Release notes](https://github.com/fastify/fastify/releases)
- [Commits](fastify/fastify@v2.13.1...v2.15.1)

Signed-off-by: dependabot[bot] <support@github.com>