Adopt "main" instead of "master" for primary git branches

[proffalken]

At present, some of the git repos still use master instead of main.

This is purely because we've not got around to changing it.

We should update the repos in line with https://github.com/github/renaming

• https://github.com/UKHackspaceFoundation/resources
• https://github.com/UKHackspaceFoundation/meetings
• https://github.com/UKHackspaceFoundation/foundation
• https://github.com/UKHackspaceFoundation/marketing
• https://github.com/UKHackspaceFoundation/constitution
• https://github.com/UKHackspaceFoundation/UKHackspaces.github.io

Task is complete when all the above have been migrated and any automated scripts etc. have been updated to pull from main rather than `master.

[proffalken]

https://github.com/UKHackspaceFoundation/website/blob/c243f761d76c89c5f07f79b667a6977437e07bdc/main/views.py#L503-L504 will also need updating