Components API deprecation - migrate all ProjectManagerListener.projectOpened() to backgroundPostStartupActivity extensions

[jwren]

See https://plugins.jetbrains.com/docs/intellij/plugin-components.html#comintellijpoststartupactivity for notice and documentation on the deprecation intentions of Components from JetBrains.

Migration forward has different directions before and after 2023.1, if we can, it would be prudent to wait until we are only supporting this major platform as a minimum version.

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• FlutterProjectStructureDetector
• [CQ] API: migrate FlutterSdkManager off projectOpened #8123

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See also: #7718

[jwren]

The pre-requisite for this was to migrate to Gradle IntelliJ platform 2.1.x to have the update verifier which is done, this work will not be finished for this upcoming Milestone, bumping to the next one.