docs: update Arc Testnet version to v0.7.3

[ibrmrdl01]

Summary

• Update the Arc Testnet version in the installation documentation from v0.6.0 to v0.7.3.
• Update the specific version installation example accordingly.

This keeps the installation documentation aligned with the current Arc Testnet release.

[osr21]

The change itself is correct — v0.7.3 is the current Arc Testnet release, and unlike the deliberately-old v0.6.0 pin in the quake README (which got reverted out of #252 for that reason), both spots in docs/installation.md genuinely should track the latest version.

But this is a byte-identical duplicate of #226 (@Sertug17, opened Aug 5 — six days before this one, same title, same file, same two hunks, even the same blob hashes 4cd389c0..6ecaa04d). I diffed both PRs via the API to confirm: there is zero delta between them. #226 is still open and mergeable, so first-come attribution applies — the fair move is to close this in favor of #226.

For context on how the docs-version cleanup is already divided up: #226 owns docs/installation.md, #238 owns the docker-compose.yml comments, and #252 owns the run-upgrade-test.sh example (a third PR, #251, was closed earlier for overlapping the first two). Between those three, every stale version reference I'm aware of is claimed — so another version-bump pass over the docs won't find open ground. If you want a contribution that lands cleanly here, fresh issues found by actually running the node or the tooling are where the unclaimed work is.