ScheduledJob - Clean up form code & improve validation

[colemanw]

Overview

Updates some old form code with better validation of job params.

Before

Api entity only validated against APIv3.
No validation of the api params input, even though it's easy to make mistakes since it's nothing but a textarea!

After

Api params are validated (at least to ensure the input is parseable)
Api entity is validated against whatever version is given in the params

Technical Details

There was no function to parse these params (because, of course there wasn't) so I extracted one from the scheduled job runner for re-use on the form.
While poking around in that CRM_Core_ScheduledJob I noticed that it relies heavily on setting undeclared class properties (watch out PHP8!) so added a few and left a TODO comment about the rest.
Removed a very large outdated comment block about APIv2.

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[seamuslee001]

Jenkins re test this please

[seamuslee001]

I tested this and confirmed it works for me