Require R >=3.5.0 and remove release_all()

[DavisVaughan]

Tied to #331, where we switch from a global preserve list to 1 preserve list per compilation unit. release_all() was intended to support experts that wanted to manually release cpp11 protected objects on R < 3.5.0, but it no longer is applicable for a few reasons:

• In the tidyverse we now require R >=3.6.0 in most places
• Using 1 preserve list per compilation unit means it will now only release objects managed by that compilation unit's preserve list
• We think the idea of potentially releasing objects you don't own can be dangerous (like if you accidentally release vroom ALTREP objects, which can persist past a .Call() boundary)

No one was using release_all() as far as we can tell.

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I've also bumped the minimum R version to >=3.5.0, so we no longer need code that supports the "old times" before we had access to R_UnwindProtect(). To this end, I've removed CPP11_USE_PRESERVE_OBJECT as well.

I've kept some details in the internals vignette, because I think the discussion of the alternative approaches is still useful for historical context.