Move GraphicsMagick to SourceForge mirrors

What does this MR do?

Move download of GraphicsMagick back to SourceForge mirrors, as we're seeing failures / HTTP 403 from existing.

• Update scripts to support pulling GraphicsMagick from SourceForge
• Add in SHA256 checksum

Changelog: changed

Impacted:

• tags/15.3.202208051520+75e4a594c14
• tags/15.3.202208051920+e3574bb04eb

Related issues

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Integration tests added to GitLab QA
• The impact any change in container size has should be evaluated