Hard-code deprecated feature flags

[jjbayer]

From Notion:

We want to avoid configuring flags in Sentry just because we need them in external Relays. Rather we want to hardcode the values in the internal Relays.

This ensures that external relays's behavior matches internal ones, and makes it unnecessary to keep feature flags around on sentry.

• Add skip_deserializing to deprecated feature flags, serialize a default
• [optional]: Stop renaming to Deprecated*
• Clean up the list (most feature flags have been deprecated >1y ago)