rpcclient: Start v9 module dev cycle.

[jholdstock]

Upcoming changes constitute breaking public API changes to the rpcclient module, therefore, this follows the process for introducing major API breaks which consists of:

• Bump the major version in the go.mod of the affected module if not already done since the last release tag
• Add a replacement to the go.mod in the main module if not already done since the last release tag
• Update all imports in the repo to use the new major version as necessary
• Make necessary modifications to allow all other modules to use the new version in the same commit
• Repeat the process for any other modules the require a new major as a result of consuming the new major(s)

[davecgh]

While I really appreciate the attention to detail in updating the docs, we wait to update those until the actual release prep commits prior to tagging because otherwise people looking at the docs will see a module that isn't available yet.

[davecgh]

Just the docs left and then it looks good. By the way, if you want to open a separate PR with the docs update (for each module since they release at different times), I'll mark them as waiting for release and it'll already be done and not waste your work that way.