Bump kinto-signer from 5.0.1 to 5.2.1 in /checks/remotesettings

Bumps [kinto-signer](https://github.com/Kinto/kinto-signer) from 5.0.1 to 5.2.1.
- [Release notes](https://github.com/Kinto/kinto-signer/releases)
- [Changelog](https://github.com/Kinto/kinto-signer/blob/master/CHANGELOG.rst)
- [Commits](Kinto/kinto-signer@5.0.1...5.2.1)

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

Author: dependabot-preview[bot]

checks/remotesettings/requirements.txt

@@ -57,9 +57,9 @@ cffi==1.12.3 \
     --hash=sha256:ed851c75d1e0e043cbf5ca9a8e1b13c4c90f3fbd863dacb01c0808e2b5204201
 pycparser==2.19 \
     --hash=sha256:a988718abfad80b6b157acce7bf130a30876d27603738ac39f140993246b25b3
-kinto-signer==5.0.1 \
-    --hash=sha256:12729c2b9857a6a7ad8320438168faa2f9c6c44e68967e7d5016a79825de9c7c \
-    --hash=sha256:df3e51fe82149aa83b3ce3d110a63f2eba125951bcca3229897a12c1ffdcf560
+kinto-signer==5.2.1 \
+    --hash=sha256:6f2f65158bb038f9bdac73e8a4bdc4f4086a014d5b78478c641cd11720a335d3 \
+    --hash=sha256:a4eb035c1d2b0ce2fdc0edaf8b334f840beeeb5a566514d89606e5505d192c59
 websockets==8.0.2 \
     --hash=sha256:049e694abe33f8a1d99969fee7bfc0ae6761f7fd5f297c58ea933b27dd6805f2 \
     --hash=sha256:73ce69217e4655783ec72ce11c151053fcbd5b837cc39de7999e19605182e28a \