BLD: Set astropy latest supported version to 6.1

[rrjbca]

Description

Set astropy latest supported version to 6.1

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request

[rrjbca]

Looks good

@jucordero I had to resolve conflicts after merging #615 so now this needs re-reviewing/approving