[embedder] Be backwards compatible when getting vkGetInstanceProcAddr

[nanokatze]

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[flutter-dashboard]

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[Hixie]

@nanokatze Did you want to land this?

[nanokatze]

I don't have any preference for either landing this or not. I opened this MR to address a concern brought up by @chinmaygarde in #39051 (comment). My personal opinion on this is that if change introduced by #39051 wasn't actually breaking anyone out there in practice, this PR should not be merged, as I don't see any point in adding compat cruft for bugs that nobody actually came to depend on.

[chinmaygarde]

Closing as this isn't a breaking change. Its possible this is confusing for implementers and we should document it at the very least.