update disclaimer message on extension docs

[aalves08]

Summary

Fixes #14953

Occurred changes and/or fixed issues

• update disclaimer message on extension docs so it's not outdated because of release of Rancher 2.10 which was done quite some time ago

Technical notes summary

• We can keep this updated message on our current v3 docs despite 2.10 being EOM (if not, very close to) and if/when we bump shell to v4 we just don't port it to the new docs OR we just kill it entirely because it's been so long since the release of 2.10

Areas or cases that should be tested

Areas which could experience regressions

Screenshot/Video

Checklist

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