Replace all mentions of "Subnet" with "Blockchain", "Chain" or "L1" equivalent

[meaghanfitzgerald]

Subnet -> L1

The developer docs language must be updated in order to reflect the nomenclature the Avalanche blockchain has adopted to describe its architecture. Changes made will be based on the context in which they exist, requiring the use of the word "blockchain" in some instances, and "L1" in others. In some instances, a "set of validators" may also be more appropriate than the former two.

[iJaack]

I helped with some context in a linked issue, hope this helps!

#1885 (comment)