[Variant] Rename ValueBuffer as ValueBuilder (#8187)

# Which issue does this PR close?

- Closes #8186

# Rationale for this change

The class has always built variant values (metadata being handled
separately), so rename it `ValueBuilder` to be more self-documenting.

# What changes are included in this PR?

Class renamed, along with all methods, local variables and documentation
that reference it.

# Are these changes tested?

It's a rename. If it compiles it's correct.

# Are there any user-facing changes?

No.

Co-authored-by: Andrew Lamb <[email protected]>

Author: scovich