Rename 'proposed' to 'staging'

To better align with standard names for this sort of thing, I would like to rename the 'proposed' -> 'staging'. I am not aware of widespread use of the current igwn-proposed environment (or versioned equivalents) so hopefully this is relatively non-disruptive.

The steps for this would be

• rename labels on this project
• rename labels on computing/sccb and update SCCB procedure docs
• update CI/docs/scripts | !796 (merged)
• rename environments with symlink from *-proposed to *-staging