Lower max relation pagination count with feature flag

What does this MR do?

As part of the investigation into https://gitlab.com/gitlab-org/gitlab/-/issues/331565 we found that counts are still relatively slow even with the 10_000 limit we have to count total number of rows.

This was originally introduced in gitlab-foss!23931 (merged).

This MR reduces that limit to 1_000.

This is behind an Ops FF, lower_relation_max_count_limit.

How will we measure succes?

The following Kibana visualisations show max/avg DB timings for 2 endpoints we investigated in https://gitlab.com/gitlab-org/gitlab/-/issues/331565

• /api/:version/projects/:id/pipelines DB timings
• /api/:version/projects/:id/jobs DB timings

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
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• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

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
• Security reports checked/validated by a reviewer from the AppSec team