Replace deprecated AWS client instantiations

[tyrannosaurus-becks]

This PR is to replace deprecated AWS client instantiations at all points where they exist in Vault. This PR has some overlap with #7738, but updates the client in more places. It doesn't really matter which of these gets merged first, probably best to merge whichever is ready first.

I ran acceptance tests locally with a valid AWS key and secret to verify that nothing was broken. I can't post the output of all tests because some of it may be sensitive. However, all tests passed and none were skipped as a result of this call:

tbex@pop-os:~/go/src/github.com/hashicorp/vault/builtin/logical/aws$ go test -v ./...

Also, the s3 tests were run and passed and are safe to post:

tbex@pop-os:~/go/src/github.com/hashicorp/vault/physical/s3$ go test -v ./...
=== RUN   TestDefaultS3Backend
--- PASS: TestDefaultS3Backend (6.70s)
=== RUN   TestS3BackendSseKms
--- PASS: TestS3BackendSseKms (6.42s)
PASS
ok  	github.com/hashicorp/vault/physical/s3	13.128s