Rename max_single_primary_size to max_primary_shard_size

[joegallo]

See #63026 (comment). max_primary_shard_size does indeed seem like a better name than max_single_primary_size to me.

Changes the naming from as added in #67705, #67842, and #68917. This'll need backporting to 7.x and 7.12.

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[joegallo]

I think I got 'em all:

joegallo@galactic:~/Code/elastic/elasticsearch $ git checkout master
Your branch is up to date with 'upstream/master'.
joegallo@galactic:~/Code/elastic/elasticsearch $ git grep -E -i '(max_single_primary_size|maxsingleprimarysize|max single)' | wc -l
     311
joegallo@galactic:~/Code/elastic/elasticsearch $ git checkout rename-max-single-primary-size
Switched to branch 'rename-max-single-primary-size'
Your branch is up to date with 'origin/rename-max-single-primary-size'.
joegallo@galactic:~/Code/elastic/elasticsearch $ git grep -E -i '(max_single_primary_size|maxsingleprimarysize|max single)' | wc -l
       0

[jakelandis]

LGTM. A quick visual inspection shows no changes but the name and correct spellings. Also, thanks for the grep output comment.

[joegallo]

BWC tests re-enabled in #69252.