feat: update docs to state no support for all projects flag

[IlanTSnyk]

• [ ] Tests written and linted ℹ︎
• [x] Documentation written in Wiki/README
• [ ] Commit history is tidy & follows Contributing guidelines ℹ︎

What this does

Updating the rEADME file to state no support for the --all-projects flag of snyk test

More info

Jira Ticket

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[aarlaud]

:tada: This PR is included in version 1.7.0 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: