Login to your Rackspace webmail account and go to contacts.
Under Export Contacts option, select Gmail and click on export button, it will ask you to save a file on your computer.
Now go to contacts.google.com and login with your Google Workspace user ID and password, you will see contacts screen like the following, from here click on Import.
You will now see a pop-up asking you to select your contacts file, click on select and choose the contacts file you exported from Rackspace.
Optional : If you want these contacts to go to a specific label (e.g old contacts), then first create a label in your Google contacts and choose that label here.
Congratulations, you have successfully migrated all your Rackspace contacts to Google Contacts.