Remove global `installedIntegrations` variable

[Lms24]

Problem Statement

While debugging a failing replay integration test in #7011, we just realized that the global installedIntegrations causes problems for the replay integration CDN bundle.

This is problematic, because the replay integration CDN bundle will get its own copy of installedIntegrations, which will always be empty as it is not populated by the SDK:

// Both bundles maintain separate copies of `installedIntegrations` and only the SDK's field will be used
SDk bundle
	sentry.init()
	sentry.addinitegration()
	installedIntegrations = [ 'Replay', 'Dedupe', ....]

replay bundle
	installedIntegrations = []
	prepareEvent() // prepareEvent accesses installedIntegrations but takes the replay bundle's version, which will always be empty

Solution Brainstorm

We should get rid of this global as this is very problematic, albeit in this particular case, only for the replay integration.
It should be good enough to use the _integrations field in BaseClient to avoid a breaking change for now and once we move to v8, we can add public getter/a cleaner way to access this field.

[github-actions]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[mydea]

I'm closing this as we're going to generally change this in v8, I don't think we'll keep this after that!