fix(deps-dev): drop usage of @ciscospark/test-users-legacy dependency

[toretre]

COMPLETES #< INSERT LINK TO ISSUE >

This pull request addresses

< DESCRIBE THE CONTEXT OF THE ISSUE >

It seems the '@ciscospark/test-users-legacy' is only referenced by 'internal-plugin-lyra' and there only for a utility method to generate a random string.
'@ciscospark/test-users-legacy' is an old dependency and might block dependency upgrades for consuming project like dependency 'requests' which abandoned and also have many CVE vulnerabilities attached.

by making the following changes

< DESCRIBE YOUR CHANGES >

1. Copy helper method to generate random string from '@ciscospark/test-users-legacy' to '@webex/test-helper-test-users/src/index.js'. Changed internal-plugin-lyra to use this moved method.
2. Drop '@ciscospark/test-users-legacy' from the package.json files where it is referenced

Change Type

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Tooling change
• Internal code refactor

The following scenarios were tested

< ENUMERATE TESTS PERFORMED, WHETHER MANUAL OR AUTOMATED >

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• This PR is related to
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  • Defect fix
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I certified that

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• All existing and new tests passed
• I have updated the documentation accordingly

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