Update outdated action versions in README examples

[Copilot]

README code examples referenced stale action versions, misleading users copying snippets into their workflows.

Changes

• actions/checkout@v2 → actions/checkout@v4
• coursier/cache-action@v6 → coursier/cache-action@v8
• coursier/setup-action@v1 → coursier/setup-action@v3

Updated in both the custom inputs example and the caching example:

steps:
  - uses: actions/checkout@v4
  - uses: coursier/cache-action@v8
  - uses: coursier/setup-action@v3

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Versions in README need to be updated</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Versions in README need to be updated #1087

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