fix(settings): specify VALID_RETENTION_DAYS for self-hosted (#6756)

Fixes getsentry/self-hosted#3421

I saw from the rust_snuba code, that we have this:
https://github.com/getsentry/snuba/blob/acfbe9a31880ecfe15888386bf40a2afe2b16088/rust_snuba/src/processors/utils.rs#L10-L20

But if we follow the default value, it will always defaults to `30`.
Whereas not everyone on the self-hosted instance uses value greater than
30. My own instance is set to 14.

https://github.com/getsentry/snuba/blob/acfbe9a31880ecfe15888386bf40a2afe2b16088/snuba/settings/__init__.py#L251

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snuba/settings/settings_self_hosted.py

@@ -7,6 +7,8 @@
 DEBUG = env("DEBUG", "0").lower() in ("1", "true")
 
 DEFAULT_RETENTION_DAYS = int(env("SENTRY_EVENT_RETENTION_DAYS", 90))
+VALID_RETENTION_DAYS = set([int(env("SENTRY_EVENT_RETENTION_DAYS", 90)), 30, 60])
+LOWER_RETENTION_DAYS = min(DEFAULT_RETENTION_DAYS, 30)
 
 REDIS_HOST = env("REDIS_HOST", "127.0.0.1")
 REDIS_PORT = int(env("REDIS_PORT", 6379))