Renamed show_subhead to show_subtitle

[kevinansfield]

closes https://linear.app/tryghost/issue/MOM-194

• whilst working on the feature our naming changed from "Subhead" to "Subtitle"
• this rename of the newsletter design setting column brings naming back into a consistent state before public release

[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

[sagzy]

LGTM 👍

[mheland]

Client had Ghost 5.83.0 install on DO Droplet, upgrade fails, possibly related?
Which intermediate version is needed to get the show_subtitle field inserted into DB?

Unable to upgrade Ghost from v5.83.0 to v5.96.0. Would you like to revert back to v5.83.0?
Message: Ghost was able to start, but errored during boot with: ALTER TABLE `newsletters` RENAME COLUMN `show_subtitle` TO `show_subhead`; - Unknown column 'show_subtitle' in 'newsletters'