gs: Fix rctx field name in LBS LNS upstream messages

[vlasebian]

Summary

The Gateway Server parsed the rctx field in the upinfo object of upstream messages of the LoRa Basics Station LNS protocol using a misspelled JSON tag (rtcx). Since Basics Station gateways send rctx (per the LNS protocol), the radio context they reported was silently ignored and the antenna index was always 0. This PR fixes the field name so the value is actually consumed.

Changes

• Fix the JSON tag of UpInfo.RCtx from rtcx to rctx.
• Update test expectations accordingly.
• Add a CHANGELOG entry.

Testing

Steps

Run the package tests:

go test ./pkg/gatewayserver/io/semtechws/lbslns/

Results

ok  go.thethings.network/lorawan-stack/v3/pkg/gatewayserver/io/semtechws/lbslns  2.732s

Regressions

Behaviour changes on multi-radio Basics Station gateways.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.

[johanstokking]

This fix is fine, but I'm not sure if this makes downlink work to other antennas than the first:

lorawan-stack/pkg/gatewayserver/io/io.go

Lines 345 to 349 in 9def57c

	if md.AntennaIndex != 0 {
	// TODO: Support downlink path to multiple antennas (https://github.com/TheThingsNetwork/lorawan-stack/issues/48)
	md.DownlinkPathConstraint = ttnpb.DownlinkPathConstraint_DOWNLINK_PATH_CONSTRAINT_NEVER
	continue
	}

Let's merge this as-is and let's continue the discussion in #48.