[Go] Update API naming to follow Golang standard pkg (1 line change)

[lwj5]

Following the naming conventions of Go standard packages, Acronyms should be all caps.

Api thus should be API instead

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[wing328]

CircleCI failure not related to this change.

[wing328]

LGTM. Let's give it a try.

In case users need to fall back to Api for easier upgrade, they can use the option apiNameSuffix (e.g. --api-name-suffix in CLI) to do that.

[wing328]

FYI. Merged #15505 to fix tests

[jablan]

@wing328 it took us quite a while to find this discussion and learn about apiNameSuffix. Any reason why it's not listed when I invoke

openapi-generator-cli config-help -g go

? Thanks!