[New Config 2] Introduce [job platforms]

[wxtim]

PR #3348 Sets out how an end point for unifying global and local configs, but contains many very complex changes.

An alternative step to making this change in a way that produces working code is to combine global and local changes whilst making the bare minimum of changes to the code-base.

This issue should cover:

• Moving settings from [runtime][job] & [runtime][remote] to a new [job platform] section.
• Adding a specialist section to the upgrader function to deal with deprecation. This is potentially controversial.

This issue should not cover:

Any changes unrelated to creation of job platforms.

[oliver-sanders]

@wxtim, @datamel closed by #3453?

[oliver-sanders]

Closed by #3453, #3772