Update permissions to relate epics

Related to #381308 (closed)

What does this MR do and why?

As part of Group minimum role should be Guest for epic rel... (&9232 - closed) we need to update permissions to relate epics, to require a Guest role instead of Reporter.

This MR updates the policy used for relating epics and the corresponding specs.

Before	After
admin_related_epic_link	admin_epic_link_relation
Permission checks: have Reporter access and related_epics feature is available for the group	Permission checks: have Guest access, can read the epic, and related_epics feature is available for the group

Before	After
admin_epic	admin_epic_relation
Permission checks: have Reporter access	Permission checks: have Guest access and can read the epic

The main reason for using a new policy instead of changing the permissions of admin_related_epic_link is to keep names consistent.

We have the existing admin_epic_link_relation that also relies on the admin_epic_relation policy, so we can group them as admin_*relation_type*_relation.

Screenshots or screen recordings

relate_epics_as_guest_user

How to set up and validate locally

Internal ednpoints (UI)

1. Logged in as an admin user create 3 epics in a group, one of them confidential
2. Create and impersonate a Guest in the group by visiting http://gdk.test:3000/admin/users
3. Visit one of epics and add link to non-confidential epic in linked epics component. This should display the epic are related and create a new system note.
4. Add link for the confidential epic, this should return an error (see screen recording above)
5. Delete the previously related epic by clicking the X button inline. The epic should be removed from the list and a new system note is created
6. Using the admin user change Guest role to Reporter and try to relate the confidential epic, it should succeed this time.

REST endpoints (link to docs)

1. Using a guest user's access token test the following endpoints

export GITLAB_PAT=<guest_user_access_token>
export GROUP_ID=<group_id>
export SOURCE_EPIC_ID=<source_epic_id>
export TARGET_EPIC_ID=<target_epic_id>

# relate source and target epics
curl --request POST --header "PRIVATE-TOKEN: $GITLAB_PAT" "http://127.0.0.1:3000/api/v4/groups/$GROUP_ID/epics/$SOURCE_EPIC_ID/related_epics?target_group_id=$GROUP_ID&target_epic_iid=$TARGET_EPIC_ID"

# list source's related epics to obtain "related_epic_link_id"
curl --request GET --header "PRIVATE-TOKEN: $GITLAB_PAT" "http://127.0.0.1:3000/api/v4/groups/$GROUP_ID/epics/$SOURCE_EPIC_ID/related_epics"

# delete related epic link
curl --request DELETE --header "PRIVATE-TOKEN: $GITLAB_PAT" "http://127.0.0.1:3000/api/v4/groups/$GROUP_ID/epics/$SOURCE_EPIC_ID/related_epics/<related_epic_link_id>"

MR acceptance checklist

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