Updated OAS spec verison to - 8.1.0

Updated OAS spec verison to - 8.1.0

feat: undone alphabetised ordering and added missing category letters.

feat: undone alphabetised ordering and added missing category letters.

chore: open api spec version updated to 7.1.0

chore: open api spec version updated to 7.1.0

Merge pull request #704 from XeroAPI/v7.0.0

OAS version update - 7.0.0

Merge pull request #625 from cristian-azocar/master

[UST-4371] Add USST fields to Invoices and Credit Notes endpoints

Merge pull request #617 from XeroAPI/OAS-6.2.0

OAS version update | 6.2.0

Merge pull request #615 from XeroAPI/OAS-6.1.1

OAS version update to 6.1.1

Merge pull request #612 from XeroAPI/OAS-6.0.1

Fix 478 | Model Syntax issue | PHP

Merge pull request #603 from andy-liu-xero/Update-add-ultimate-sub-sc…

…riptions

Added ultimate sub plans.

Merge pull request #601 from ronnel-santiago-xero/apib-5555_pageinfo_fix

[APIB-5555] Use the correct ref for bank transaction, contacts, credi…