Update PostgreSQL Datastore documentation for Postgre-15 compatibility

[lspxxv]

Description

We need to update postgres documentation here to include running the command below that will grant all privileges to the sensu user for the sensu_events db

Category

This is an issue with:

• [ ] errata (error in doc content)
• [ ] new doc (requires a completely new doc)
• [x] update (add missing or refresh existing content)
• [ ] improvement (improve docs content or format)

Affected pages

https://docs.sensu.io/sensu-go/latest/operations/deploy-sensu/scale-event-storage/

Expected behavior

Docs should include the command - GRANT ALL ON SCHEMA public TO sensu - for PostgreSQL-15 compatibility.

Current behavior

The current installation/configuration steps would only work for earlier versions of postgres

Corrections needed

Add GRANT ALL ON SCHEMA public TO sensu while interacting with the sensu_events db.

Error when necessary privileges are not applied.

"component":"ent-store","error":"error migrating database to latest version: Could not get DB version: ERROR: permission denied for schema public (SQLSTATE 42501)","level":"error","msg":"error opening postgres store, retrying..

[jhenderson-pro]

Confirmed the same behavior and the fix.