Update lndclient branch `lnd-18-4` to track lnd `v0.18.4-beta` release

[dstadulis]

Updating lndclient branch lnd-18-4 to point to the tagged version of lnd is a dependency for the custom channel saga and needs to be completed before merging [the mother of all PRs](link to the original PR).

To complete this task:

• 1. Tag / push lnd v0.18.4-beta to https://github.com/lightningnetwork/lnd/ tagged (at least an RC tag)
• 2. Update the lndclient branch lnd-18-4 to point to the tagged version of lnd

This update is a prerequisite for the litd version (to be released with custom-channels code merged into main) dependency chain.

[guggero]

Fixed by https://github.com/lightninglabs/lndclient/releases/tag/v0.18.4-6.