Enable cached_sidebar_open_issues_count FF by default

What does this MR do?

Related to #323493 (closed)

Enable by default cached_sidebar_open_issues_count feature flag introduced in !49739 (merged)

• Current status: enabled gloablly since Jan 22 (internal link).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
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• Style guides
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• 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• 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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