Docs: Note experimental support for yarn

What does this MR do?

Fix for #327160 (closed)

Add a note that the License Compliance tool supports only npm lock file version 1, meaning that LicenseFinder supports only npm version 7 and earlier.

Screenshots or Screencasts (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

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Security

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
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