docs: mark separated-host RPC transport deprecated as of v0.8.0

[kutluhaneth46]

Summary

Updates the separated-hosts warning in docs/running-an-arc-node.md: the RPC/HTTP Engine API transport is deprecated as of v0.8.0 (released), not will be deprecated.

Fixes #294

Test plan

• Doc-only change; wording matches current release state
• Removal timeline (v0.9.0) unchanged

[osr21]

Verified against main (66ad2d5): the diff applies to the current line 408 and the new wording keeps the v0.9.0 removal clause — the operationally important half for anyone running separated hosts today. Also confirmed the one-line scope is complete as-is: this was the only stale future-tense instance in the repo (line 789 of the same file already reads "HTTP/RPC transport (deprecated):"), and the admonition's remaining claims match the shipped v0.8.0 binary — crates/malachite-cli/src/cmd/start.rs emits the DEPRECATION: startup warning whenever an RPC transport option is set, with unit tests covering both polarities. Nothing else in the warning needs touching.

Two housekeeping notes:

• mergeable_state: blocked here is just first-time-contributor CI awaiting maintainer approval, as with other external PRs on this repo (e.g. docs: correct the v0.8.0 insufficient-balance RPC error-text matrix #291) — nothing to do on your side.
• For the maintainers: the docs: separated-host warning still says RPC transport will be deprecated in v0.8.0 #294 reporter (@Xelp66) had offered to submit this fix and asked for assignment there. Flagging the overlap so credit and closure can be handled as you see fit — the diff matches what the issue proposed, with the removal clause retained per the follow-up discussion in docs: separated-host warning still says RPC transport will be deprecated in v0.8.0 #294.

[melekes]

good catch 👍 thanks

[osr21]

@melekes Im available to offer contributions and support ahead of mainnet.