tests: Pin api submodule to compatibile version for upgrade tests

[eculver]

Description

This is an alternative to #15827. The details are in #15827 but the gist is that because the upgrade tests use the latest version of the api submodule, the test code will fail when running against a Consul 1.14 container instance because it is sending a config field that is not known. I was able to pull in a compatible api submodule using a replace directive. It seems to work, so I'm wondering if we should use this instead of removing the test code that triggers the compatibility issue.

Testing & Reproduction steps

cd test/integration/consul-container
go test -run TestPeering_UpgradeToTarget_fromLatest ./... --target-version local --latest-version latest

[eculver]

I don't know why the line above does not result in api/v1.18.0 from being used, but it doesn't, the replace directive appears to be required.

[huikang]

@eculver , there is another alternative: I think in Go the json tag will ignore ",omitempty" if it is a struct. So we may need to change all of them to pointer like *TransparentProxyConfig

consul/api/config_entry.go

Lines 294 to 298 in ab199a1

	TransparentProxy *TransparentProxyConfig `json:",omitempty" alias:"transparent_proxy"`
	Config map[string]interface{} `json:",omitempty"`
	MeshGateway MeshGatewayConfig `json:",omitempty" alias:"mesh_gateway"`
	Expose ExposeConfig `json:",omitempty"`
	AccessLogs AccessLogsConfig `json:",omitempty"`

Please see #15831

[eculver]

Closing this in favor of #15831.