Update serverless restrictions wrt proxy support

[kilfoyle]

Agent and Fleet Server proxies are now supported on serverless. This updates the serverless restrictions page to clarify the current limitations.

@jillguyonnet Thanks for the super clear issue! Please let me know if I've missed anything. One small thing I wonder is if "Outputs" should maybe be "Elastic Agent outputs", or instead maybe just a sub-heading of the "Elastic Agent" section.

Closes: #887

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[jillguyonnet]

Thanks @kilfoyle! I agree with your suggestion, a sub-heading of "Elastic Agents" sounds great.

[amyjtechwriter]

LGTM