Bump fastify from 2.13.0 to 2.15.1

Bumps [fastify](https://github.com/fastify/fastify) from 2.13.0 to 2.15.1.
- [Release notes](https://github.com/fastify/fastify/releases)
- [Commits](fastify/fastify@v2.13.0...v2.15.1)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

package-lock.json

@@ -7,7 +7,7 @@
         "@slack/web-api": "^5.8.0",
         "cross-env": "^7.0.2",
         "dotenv": "^8.2.0",
-        "fastify": "^2.13.0",
+        "fastify": "^2.15.1",
         "fastify-blipp-log": "^1.0.2",
         "fastify-helmet": "^3.0.2",
         "fastify-static": "^2.6.0",