InjectedMetadataSetup to be removed by 8.8

[TinaHeiligers]

InjectedMetadataSetup is marked as deprecated to be removed by 8.8. However, remaining consumers are blocking the final removal:

• monitoring

getInjected: (name: string, defaultValue?: unknown): string | unknown =>
    core.injectedMetadata.getInjectedVar(name, defaultValue),

• fleet:

injectedMetadata: {
    getInjectedVar: () => null,
},

Consumers need to migrate away from the deprecated API for Core to remove the API.

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

I think we should be able to manage ourselves, none of the remaining usages seems to be, well, used.

• fleet

kibana/x-pack/plugins/fleet/.storybook/context/index.tsx

Lines 85 to 87 in 27ac4fc

	injectedMetadata: {
	getInjectedVar: () => null,
	},

I couldn't find any usage of it, and the fact that it always return nil would tend to confirm that we can just delete it.

• monitoring

kibana/x-pack/plugins/monitoring/public/legacy_shims.ts

Lines 94 to 95 in bebec37

	getInjected: (name: string, defaultValue?: unknown): string | unknown =>
	core.injectedMetadata.getInjectedVar(name, defaultValue),

Same, grepping in the plugin's code, I can only find 3 usages of getInjected, which are just the definition. I think we should be able to just delete the definition.