docs: update Arc Testnet version to v0.7.3 in installation.md (#226)

## Summary

The Versions table in `docs/installation.md` listed `v0.6.0` as the
required version for Arc Testnet. The current release is `v0.7.3`.

## Problem

A node operator following the installation guide would install `v0.6.0`,
which can no longer sync with Arc Testnet. Per `BREAKING_CHANGES.md`:

> "testnet node operators must use v0.7.2 before timestamp 1781791200
(2026-06-18 14:00:00 UTC), when Zero7 activates on testnet. Earlier
versions are not supported."

That deadline has passed — `v0.6.0` nodes are incompatible with the
current testnet network.

## Change

- `docs/installation.md`: Versions table updated from `v0.6.0` →
`v0.7.3`

Closes #225

Author: Sertug17

docs/installation.md

@@ -15,7 +15,7 @@ Consult the table below to confirm which version to run for each network.
 
 | Network     | Version |
 |-------------|---------|
-| Arc Testnet | v0.6.0  |
+| Arc Testnet | v0.8.0  |
 
 ## Pre-built Binary
 
@@ -61,7 +61,7 @@ source "$ARC_HOME/env"
 To install a specific version, run `arcup` with `--install`:
 
 ```sh
-arcup --install v0.6.0
+arcup --install v0.8.0
 ```
 
 Next, verify that the three Arc binaries are installed: