docs: update README to actions/cache@v6

[MikeMcC399]

Situation

• PR chore(deps): update actions/cache action to v6 #1812 bumped to actions/cache@v6 in the workflow example-install-only.yml only, without updating README > Install Cypress only.

This is outside the capabilities of Renovate and has to be handled manually.

Change

• Update README > Install Cypress only to actions/cache@v6

---

Note

Low Risk
Documentation-only change to a README code sample; no CI or application code is modified.

Overview
Aligns the Install Cypress only README workflow snippet with the repo’s live example workflow by bumping actions/cache from v5 to v6.

No runtime or action behavior changes—only the documented copy-paste YAML for manual Cypress-only install + cache.

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[cypress-app-bot]

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[MikeMcC399]

Obsolete due to #1838