Update purchase DB sharding key docs

Update the DB docs with the current sharding key plans for the purchase
related tables: upcoming_reconciliations, gitlab_subscription_histories.

MR: !152752

db/docs/gitlab_subscription_histories.yml

@@ -8,4 +8,4 @@ description: History log for the gitlab_subscriptions table
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/19694
 milestone: '12.6'
 gitlab_schema: gitlab_main_cell
-sharding_key_issue_url: TODO
+sharding_key_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/462598

db/docs/upcoming_reconciliations.yml

@@ -8,4 +8,4 @@ description: Stores the data needed to notify a user of an upcoming reconciliati
 introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/63054
 milestone: '14.0'
 gitlab_schema: gitlab_main_cell
-sharding_key_issue_url: TODO
+sharding_key_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/462598