feat(deprecation): remove 3.6 deprecated features for 4.0

[Joibel]

Motivation

As part of 3.6 we deprecated 4 features:

• podPriority doesn't work any modern kubernetes and has been deprecated for a while
• schedule is replaced by schedules in cron workflows - this is a trivial update to make to the yaml
• mutex and semaphore are replaced by the pluralised version - this is a trivial update to make to the yaml

Removing legacy code reduces maintainer burden and the chance for errors.

Modifications

Removed the features mentioned.
Removed podPriority tests.
Updated a message about use of schedule and schedules at the same time, and fixed the test. Code needed moving slightly to make a test case pass as the validator only returns one error.
Otherwise Updated all tests which used these features to use the new version.
/util/deprecations is unused at the moment, so the tests are a bit meaningless. I propose to deprecate onExit using this mechanism, but in a separate PR.

Verification

Relying on updated tests.

Ran the UI and checked the Cron Workflows list view and editor.

Documentation

Documentation for this is mostly auto-generated, but has been updated where necessary.

[MasonM]

Mentioned in the meeting, but the UI changes for the schedule deprecation overlaps with #14197, which handles transparently upgrading schedule to schedules:

argo-workflows/ui/src/shared/services/cron-workflow-service.ts

Lines 5 to 17 in e9e7c43

	// Handle CronWorkflows using the deprecated "schedule" field by automatically
	// migrating them to use "schedules".
	// Also, gracefully handle invalid CronWorkflows that are missing both
	// "schedule" and "schedules".
	function normalizeSchedules(cronWorkflow: any): CronWorkflow {
	cronWorkflow.spec.schedules ??= [];
	// TODO: Delete this once we drop support for "schedule"
	if ((cronWorkflow.spec.schedule ?? '') != '') {
	cronWorkflow.spec.schedules.push(cronWorkflow.spec.schedule);
	delete cronWorkflow.spec.schedule;
	}
	return cronWorkflow as CronWorkflow;
	}

That's now been merged, hence the conflicts.