Consistently refer to Key Value secrets engine as KV

[hellobontempo]

Historically we've been inconsistent across Vault products referring to the Key Value secrets engine. See one example in the docs below where there are three different styles of key value:

In a Vault UI sync meeting last week we discussed with design and the core PM to come to a consensus on how to refer to the Key Value secrets engine. We settled on Key Value as the complete term and KV version 1 (KV v1) or KV version 2 (KV v2), without the slash (K/V) because it's an acronym for the secrets engine. This PR updates references to K/V and Key/Value

[github-actions]

CI Results:
Failures:

Test Type	Package	Test	Logs
race	vault	TestOIDC_PeriodicFunc	view test results
race	vault	TestOIDC_PeriodicFunc/test-key-nil-next-signing-key	view test results

[github-actions]

Build Results:
All builds succeeded! ✅

[yhyakuna]

Consul docs also use KV instead of K/V, so I appreciate this change for better consistency. 👍

[raskchanky]