Added composite index to posts table for type,status

[9larsons]

ref https://linear.app/tryghost/issue/CFR-35

• performance improvement intended for the content api/get helpers

The posts table is shared by posts and pages and seldom is queried for both. It makes sense to add an index on type, and from the perspective of the content API, also on status as you're almost only ever querying for published posts or published pages.

[github-actions]

It looks like this PR contains a migration 👀
Here's the checklist for reviewing migrations:

General requirements

• Satisfies idempotency requirement (both up() and down())
• Does not reference models
• Filename is in the correct format (and correctly ordered)
• Targets the next minor version
• All code paths have appropriate log messages
• Uses the correct utils
• Contains a minimal changeset
• Does not mix DDL/DML operations

Schema changes

• Both schema change and related migration have been implemented
• For index changes: has been performance tested for large tables
• For new tables/columns: fields use the appropriate predefined field lengths
• For new tables/columns: field names follow the appropriate conventions
• Does not drop a non-alpha table outside of a major version

Data changes

• Mass updates/inserts are batched appropriately
• Does not loop over large tables/datasets
• Defends against missing or invalid data
• For settings updates: follows the appropriate guidelines

[9larsons]

Index is added in a few ms for a test set of 20k posts. This may be a bit longer for the couple biggest customers - I think the largest has ~125k posts, but all in all is very quick.

[daniellockyer]

Version needs bumping