Add deprecation note for workspaceSubmissionStats (WOR-1568).

[cahrens]

Related to broadinstitute/rawls#2791

Have you read CONTRIBUTING.md lately? If not, do that first.

I, the developer opening this PR, do solemnly pinky swear that:

• I've followed the instructions if I've made any changes to the API, especially if they're breaking changes
• I've updated the RC_XXX release ticket with any manual steps required to release this change
• I've updated the FISMA documentation if I've made any security-related changes, including auth, encryption, or auditing

In all cases:

• Get two thumbsworth of review and PO signoff if necessary
• Verify all tests go green
• Squash and merge; you can delete your branch after this unless it's for a hotfix. In that case, don't delete it!
• Test this change deployed correctly and works on dev environment after deployment