BlockCanvas: Make contentRef property stable

[costasovo]

What?

This PR makes BlockCanvas's contentRef property stable.

Why?

It is used by a couple of custom build block editors. Please see more details in https://github.com/WordPress/gutenberg/issues/67209

Fixes https://github.com/WordPress/gutenberg/issues/67209

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Co-authored-by: costasovo <costasovo@git.wordpress.org>

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