Specify primary key for tables `elasticsearch_indexed_*`

These two tables don't have a primary key defined:

1. elasticsearch_indexed_namespaces
2. elasticsearch_indexed_projects

Additionally, they don't have a suitable unique index defined which we'd be able to promote to a primary key (like done in !45198 (merged)). That is, the relevant columns are not marked NOT NULL.

The goal here is to define a primary key for both tables. This is important e.g. for logical replication to succeed, but also a requirement for tables in general.

We may want to check if we can mark the column covered by the unique index as not null and then promote the unique index to the primary key.

TODOs

• Determine owner for this issue
• Gather statistics on those tables