Increase default FrontendMaxNamespaceVisibilityRPSPerInstance

[rodrigozhou]

What changed? Increase default value of frontend.namespaceRPS.visibility to 50.

Why? Unit tests TestIntegrationSuite/Test_DeleteWorkflowExecution_* were failing with error namespace rate limit exceeded, so I'm increasing the default limit.

How did you test it? Running unit tests locally consistently passed without rate limit exceeded error.

Potential risks It might overload the system?

Is hotfix candidate? No.

[yiminc]

If this is just for test, could you overwrite the config value in the test?