v0.2.3 - CI, gofmt-clean generated code, docs

Tooling and documentation release - no API or behavior change, and no
schema change (still XAPI release 26.16.1-next,
xenapi.SchemaXAPIRelease unchanged).

CI

• New Tests workflow: go build, go vet and go test -short -race on
  every push and pull request, across a matrix of Go 1.16 (the minimum
  go.mod declares) and current stable Go.
• A repository-wide gofmt check runs alongside it.
• regenerate.yml now pins the generators to stable Go instead of
  following go.mod's 1.16. The generators format their own output, so
  the toolchain running them decides what the committed files look like,
  and gofmt's rules change between Go releases (Go 1.19 reformatted doc
  comments). The 1.16 minimum for consumers of the library is covered by
  the test matrix instead.

Generated code

• xenapi.go and gen_errors.go render into memory and run the result
  through go/format before writing, so *_gen.go and error.go are
  gofmt-clean by construction rather than depending on whitespace tuning
  in the templates.
• Everything was regenerated accordingly. The diff is whitespace-only:
  apart from intra-line spacing (_method + " -> " became
  _method+" -> "), the only change is the blank // line gofmt inserts
  before an indented block in a doc comment. No declaration, signature or
  value changed.

Documentation

• New UPDATES_AND_PATCHES.md
  • finding pending patches: unapplied pool_update objects,
    repository-based updates, and outstanding post-update guidances.
• GOOD_TO_KNOW.md: the other_config boot_time/agent_start_time
  convention.
• MAINTAINING.md: a section on formatting of generated code and why
  regeneration needs a current toolchain.
• README: Go Reference, release, Go version, license and CI badges.
• Code of Conduct, issue templates, and a pull request template.
• LICENSE copyright holder corrected.