BufferGeometryUtils: Simplify function names

[WestLangley]

BufferGeometryUtils.interleaveAttributes() exists, so the following changes are consistent with current nomenclature:

mergeBufferGeometries() -> mergeGeometries()
mergeBufferAttributes() -> mergeAttributes()

[WestLangley]

I will do necessary clean-up of warnings in a follow-on PR.

[Benjythebee]

When this PR was merged, it should have been followed up with a @deprecated use '...' flag on the previous name functions;
Without the flag, a dev can end up super confused why his/her IDE is screaming "function does not exists" when updating Three

[WestLangley]

@Benjythebee

The deprecation warnings were included in this PR, and in the migration guide.

Deprecation warnings are removed after 10 releases, as per the policy stated at the top of the migration guide.