remove 3.0 iso links to avoid confusion with people interested in 4.0

[jperrin]

Merge Checklist

All boxes should be checked before merging the PR (just tick any boxes which don't apply to this PR)

• The toolchain has been rebuilt successfully (or no changes were made to it)
• The toolchain/worker package manifests are up-to-date
• Any updated packages successfully build (or no packages were changed)
• Packages depending on static components modified in this PR (Golang, *-static subpackages, etc.) have had their Release tag incremented.
• Package tests (%check section) have been verified with RUN_CHECK=y for existing SPEC files, or added to new SPEC files
• All package sources are available
• cgmanifest files are up-to-date and sorted (./cgmanifest.json, ./toolkit/scripts/toolchain/cgmanifest.json, .github/workflows/cgmanifest.json)
• LICENSE-MAP files are up-to-date (./LICENSES-AND-NOTICES/SPECS/data/licenses.json, ./LICENSES-AND-NOTICES/SPECS/LICENSES-MAP.md, ./LICENSES-AND-NOTICES/SPECS/LICENSE-EXCEPTIONS.PHOTON)
• All source files have up-to-date hashes in the *.signatures.json files
• sudo make go-tidy-all and sudo make go-test-coverage pass
• Documentation has been updated to match any changes to the build system
• Ready to merge

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Summary

This PR removes 3.0 iso information from the main readme to avoid confusion for users looking for 4.0 content. It leaves iso info for 3.0 in the readme-3.0 to ensure it's still available.

Change Log

• Updated README to remove iso confusion

Does this affect the toolchain?

NO

Associated issues

• #xxxx

Links to CVEs

• https://nvd.nist.gov/vuln/detail/CVE-YYYY-XXXX

Test Methodology

• Pipeline build id: xxxx