ISSUE-380: 99designs/keyring dependency causes build warnings on macOS Catalina

[sijie]

Original Issue: apache#380

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github.com/apache/pulsar-client-go/oauth2/store depends on 99designs/keyring v1.1.5, which in turn depends on an older version of github.com/keybase/go-keychain, which throws deprecation warning error messages when building on MacOS. v1.1.6 includes a fix for this, though it uses a replace directive to point to a fork of go-keychain, which itself is somewhat out of date:
99designs/keyring#66

Consider updating to v1.1.6 and keeping this bug open to track the dependency. When it no longer requires a replace directive, it is probably fine to close the issue.