chore(deps): update dependency slackeventsapi to v2.2.0 (#4149)

Author: renovate-bot

functions/slack/requirements.txt

@@ -1,3 +1,3 @@
 google-api-python-client==1.9.3
 flask==1.1.2
-slackeventsapi==2.1.0
+slackeventsapi==2.2.0