Mantid Imaging 2.8 Release

[samtygier-stfc]

• [x] Make sure release instructions are in shared folder and up to date ("Versions" needs slight alteration - sub-point 4 is out of date)
• [x] Release notes #2256
• [x] Versions #2255
• [x] New authors
• [x] Create release branch
• [x] Create release
• [x] IDAaaS (move stable branch)
• [x] Versions on main

[samtygier-stfc]

Commits/PR to cherrypick to main

Some of #2255

2258

2260

2256

2264

[samtygier-stfc]

All done https://github.com/mantidproject/mantidimaging/releases/tag/2.8.0