Replace deprecated gateway4 with routes syntax in netplan examples

[berendt]

The gateway4 key is deprecated in netplan and causes errors on Ubuntu 24.04. Use the routes syntax instead, consistent with the network configuration guide. (osism/issues#1334)

Closes osism/issues#1334

AI-assisted: Claude Code

[github-actions]

❌MegaLinter analysis: Error

Descriptor	Linter	Files	Fixed	Errors	Warnings	Elapsed time
✅ ACTION	actionlint	3		0	0	0.03s
✅ JSON	jsonlint	4		0	0	0.1s
✅ JSON	prettier	4		0	0	0.45s
✅ JSON	v8r	4		0	0	8.58s
✅ MARKDOWN	markdownlint	154		0	0	2.0s
✅ MARKDOWN	markdown-table-formatter	154		0	0	0.28s
✅ REPOSITORY	checkov	yes		no	no	17.19s
✅ REPOSITORY	git_diff	yes		no	no	0.07s
✅ REPOSITORY	secretlint	yes		no	no	1.83s
✅ REPOSITORY	trufflehog	yes		no	no	4.25s
✅ SPELL	cspell	183		0	0	5.32s
❌ SPELL	lychee	163		1	0	36.51s
✅ YAML	prettier	5		0	0	0.35s
✅ YAML	v8r	5		0	0	6.48s
✅ YAML	yamllint	5		0	0	0.42s

Detailed Issues

❌ SPELL / lychee - 1 error

[ERROR] https://cloud-init.io/ | Too many redirects: error following redirect for url (https://cookies.canonical.com/api/v1/cookies/session?return_uri=https%3A%2F%2Fcloud-init.io%2F_cookies%2Fcallback%3Fcode%3D958859f82a1b23545bfe681392d65420%26action%3DNone%26cookie_redirect_success%3Dtrue%26return_uri%3Dhttps%3A%2F%2Fcloud-init.io%2F_cookies%2Fcallback%3Fcode%253D12284b7462390afe120f64b397a2eee2%2526action%253DNone%2526cookie_redirect_success%253Dtrue%2526return_uri%253Dhttps%3A%2F%2Fcloud-init.io%2F%2526preferences_unset%253Dtrue%26preferences_unset%3Dtrue)
📝 Summary
---------------------
🔍 Total..........775
✅ Successful.....732
⏳ Timeouts.........0
🔀 Redirected.......0
👻 Excluded........42
❓ Unknown..........0
🚫 Errors...........1

Errors in docs/cloud-in-a-box/index.md
[ERROR] https://cloud-init.io/ | Too many redirects: error following redirect for url (https://cookies.canonical.com/api/v1/cookies/session?return_uri=https%3A%2F%2Fcloud-init.io%2F_cookies%2Fcallback%3Fcode%3D958859f82a1b23545bfe681392d65420%26action%3DNone%26cookie_redirect_success%3Dtrue%26return_uri%3Dhttps%3A%2F%2Fcloud-init.io%2F_cookies%2Fcallback%3Fcode%253D12284b7462390afe120f64b397a2eee2%2526action%253DNone%2526cookie_redirect_success%253Dtrue%2526return_uri%253Dhttps%3A%2F%2Fcloud-init.io%2F%2526preferences_unset%253Dtrue%26preferences_unset%3Dtrue)

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