Support Go 1.21

[bestbeforetoday]

As a Go application developer I want to build and run my application with Go 1.21 So that I can stay current with latest features and bug fixes

Go 1.21 was released on 8 August 2023. Once available, the following tasks are required:

• [x] Add Go 1.21 to GitHub Actions test matrix
• [x] Use Go 1.21 as the default Go version for version-agnostic GitHub Actions workflows
• [x] Either on next release or if no code changes are required to pass tests, update documentation to include Go 1.21 in compatibility matrix.