Release/1.21.0

[JamesMBligh]

Release checklist:

• [x] Has version number in markdown been changed?
• [x] Is publish date accurate?
• [x] Has version number in swagger files been changed?
• [x] Have the previous release diff statements been removed?
• [x] Have new diff statements for this release been created?
• [x] Have release notes been created?
• [x] Are release notes complete and accurate?
• [x] Has archive been created?
• [x] Does archive have banner indicating it isn’t current?
• [x] Has link to latest for new version been created in archive repository?
• [x] Is archive linked in markdown?
• [x] Have the swagger markdown files been generated successfully?
• [x] Have the distribution files been generated successfully?
• [x] Has a full review of all changes been conducted?