[Feature flag] Enable pagination token exact match

<!-- Title suggestion: [Feature flag] Enable description of feature -->

## What

Enable the `exact_pagination_token_match` feature flag ...

## Owners

* Team: ~"group::source code"
* Most appropriate slack channel to reach out to: `#g_create_source-code-be`
* Best individual to reach out to: @vyaklushin

## Expectations

### What release does this feature occur in first?

https://gitlab.com/gitlab-org/gitaly/-/commit/69eac9a0db256b27a8872f6e871e5d7e61485471 
14.3.0-rc42



### What are we expecting to happen?

Gitaly will return an error when page token is not found.

Previously, it would try to return the next element of the collection that goes after the unknown page token. 

### What might happen if this goes wrong?

Users can see an error when requesting a list of branches.

### What can we monitor to detect problems with this?

https://log.gprd.gitlab.net/goto/9e72ab5de811c34415eb741c3696eaff

## Beta groups/projects

If applicable, any groups/projects that are happy to have this feature turned on early. Some organizations may wish to test big changes they are interested in with a small subset of users ahead of time for example.

- `gitlab-org/gitlab` / `gitlab-org/gitaly` projects
- `gitlab-org`/`gitlab-com` groups
- ...

## Roll Out Steps

- [x] [Read the documentation of feature flags](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#feature-flags)
- [x] Add ~"featureflag::staging" to this issue ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#feature-flag-labels))
- [x] Is the required code deployed? ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#is-the-required-code-deployed))
- [x] Do we need to create a [change management issue](https://about.gitlab.com/handbook/engineering/infrastructure/change-management/#feature-flags-and-the-change-management-process)? ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#do-we-need-a-change-management-issue))
- [x] Enable on staging ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#enable-on-staging))
- [x] Test on staging ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#test-on-staging))
- [x] Verify the feature flag was used by checking Prometheus metric [`gitaly_feature_flag_checks_total`](https://prometheus.gstg.gitlab.net/graph?g0.expr=sum%20by%20(flag)%20(rate(gitaly_feature_flag_checks_total%5B5m%5D))&g0.tab=1&g0.stacked=0&g0.range_input=1h)
- [x] Announce on this issue an estimated time this will be enabled on GitLab.com
- [x] Add ~"featureflag::production" to this issue
- [x] Enable on GitLab.com by running chatops command in `#production` ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#enable-in-production))
- [x] Cross post chatops slack command to `#support_gitlab-com` and in your team channel
- [x] Verify the feature flag is being used by checking Prometheus metric [`gitaly_feature_flag_checks_total`](https://prometheus.gprd.gitlab.net/graph?g0.expr=sum%20by%20(flag)%20(rate(gitaly_feature_flag_checks_total%5B5m%5D))&g0.tab=1&g0.stacked=0&g0.range_input=1h)
- [x] Announce on the issue that the flag has been enabled
- [x] Did you set the feature to both `100%` **and** `true` ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#enable-in-production))
- [x] Submit a MR to have the feature `OnByDefault: true` and add changelog entry ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#feature-lifecycle-after-it-is-live))
- [x] Have that MR merged
- [x] Possibly wait for at least one deployment cycle ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#two-phase-ruby-to-go-rollouts))
- [x] Submit an MR to remove the pre-feature code from the codebase and add changelog entry ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#feature-lifecycle-after-it-is-live))
- [x] Have that MR merged
- [x] Remove the feature flag via chatops ([howto](https://gitlab.com/gitlab-org/gitaly/-/blob/master/doc/PROCESS.md#remove-the-feature-flag-via-chatops))
- [x] Close this issue

Story points: 1.0