Removes server name checking from secret-based registry auth

[sonnysideup]

If a user wants to authenticate with 2 or more registries contained within a single k8s secret, then they are required to create an ImageBuild with the following auth section:

registryAuths:
- server: "server1"
  secret:
    name: many-registry-auths
    namespace: ns
- server: "server2"
  secret:
    name: many-registry-auths
    namespace: ns

and this can become somewhat cumbersome as that list grows. Furthermore, the server field for secrets is meant to server as a verification mechanism (i.e. an error will be raised if it's missing from the underlying secret data). I think we can add the verification to another field, or implement a different way that removes the labor involved here. For now, I'm disabling it and promoting all auths contained within the secret over to buildkit.

Tested locally pulling from one private registry and pushing to a different private registry with a single secret containing both sets of credentials.

/cc @ddl-kevin

[ddl-kevin]

LGTM

[saahildhulla]

👍