[APM] Deprecate `apm_oss` app

[sorenlouv]

The apm_oss app is no longer needed since everything in Kibana is now under the same license.
We should therefore deprecate all the options in apm_oss and replace them with identical options in xpack.apm.

Dure to backwards compatibility we'll have to keep the old settings around for the foreseeable future but with deprecation warnings.

kibana/src/plugins/apm_oss/server/index.ts

Lines 15 to 28 in e86d909

	export const config = {
	schema: schema.object({
	enabled: schema.boolean({ defaultValue: true }),
	transactionIndices: schema.string({ defaultValue: 'apm-*' }),
	spanIndices: schema.string({ defaultValue: 'apm-*' }),
	errorIndices: schema.string({ defaultValue: 'apm-*' }),
	metricsIndices: schema.string({ defaultValue: 'apm-*' }),
	sourcemapIndices: schema.string({ defaultValue: 'apm-*' }),
	onboardingIndices: schema.string({ defaultValue: 'apm-*' }),
	indexPattern: schema.string({ defaultValue: 'apm-*' }),
	fleetMode: schema.boolean({ defaultValue: true }),
	}),
	deprecations,
	};

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)