Update USP docs to match change to default keepalive

[Morgan-P-04]

Proposed changes

This PR updates the default keepalive connections value for the Upstream Settings Policy API, to match recent changes. The change removes the outdated default previously used and updates the configuration examples to reflect the new changes.

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[sjberman]

Be sure to target the ngf-release-2.7 branch instead of main for this

[bjee19]

@Morgan-P-04

Could we adjust this to look something like

By default, the keepAlive directive is enabled with a value of 32. You can override this value or disable keepAlive entirely by configuring an UpstreamSettingsPolicy. To disable keepalive, set the connections field to 0.

The following example creates an UpstreamSettingsPolicy that configures keepalive connections for the coffee Service with a value of 24:

To still tell the user what the default keepAlive is and to show how to set the value to something different?

[bjee19]

Or the first line could be

By default, the keepAlive directive is omitted, causing NGINX to default to the keepalive value 32. You can override this value or disable keepAlive entirely by configuring an UpstreamSettingsPolicy. To disable keepalive, set the connections field to 0.

and we can remove the line further down

If keepAlive.connections is omitted, the NGINX default value for the keepalive directive is used. To disable keepalive, set keepAlive.connections to 0.