Rename KeepAlivePingInterval to KeepAlivePingDelay

[JamesNK]

SocketsHttpHandler is adding KeepAlivePingDelay property. Our setting is called KeepAlivePingInterval.

dotnet/runtime#31198 (comment)

I think both names are fine. For consistency let rename Kestrel's property to match.

[davidfowl]

So we expose any other intervals?

[JamesNK]

Everywhere we expose TimeSpan properties on KestrelServerOptions/KestrelServerLimits/Http2Limits the property name ends with Timeout.

[ghost]

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