[DOC] release notes for 3.31.0

[TASK] bump version number to 3.31.0
[FIX] Correct model direction validation and tests
[FIX] remove default value of 'invoice_id' from model
Merge pull request #432 from ripple/release-3.31.0-rc1

Release 3.31.0 rc1
[TASK] Migrate apiRouter to express.Router.
[FEATURE] Extract user routes into express.Router.