Remove epics_update_dates_upstream feature flag

What does this MR do?

Remove feature flag check for epics dates update for parents. Given this feature flag was set to default true and it was never disabled, we need to just remove one line of code.

explain SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'epics_update_dates_upstream'

Index Scan using index_feature_gates_on_feature_key_and_key_and_value on public.feature_gates  (cost=0.27..4.29 rows=1 width=63) (actual time=2.123..2.123 rows=0 loops=1)
   Index Cond: ((feature_gates.feature_key)::text = 'epics_update_dates_upstream'::text)
   Buffers: shared read=2
   I/O Timings: read=2.069

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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re #36393 (closed)