fix: only `generate` on latest Go version

As per oapi-codegen/oapi-codegen#2536, the
generated code has changed on Go 1.27.

Instead of having inconsistently generated code, we can move to only
generate on the latest version of Go (Go 1.27, `stable`).

fix(generate): show full diff when `make generate` fails

So it's easier to debug, we can show a full diff, while making sure to
return an exit code as we expect.

feat: test against Go 1.27

Now it's available.

fix: run `tidy-ci` instead of `tidied`

As noted in oapi-codegen/oapi-codegen#2372,
otherwise this leads to cases where we're not correctly `go mod tidy`ing
across the project.

As all our projects have a `tidy-ci`, we can use that task.

ci: add Release Drafter

To automagically generate release notes.

chore: Update CI Go versions

With Go 1.26 releasing, we're advancing our supported Go versions
to [1.24, 1.25, and 1.26]. I removed "stable" and "oldstable" since
they overlap with "1.26" and "1.25", respectively, and it doesn't
make sense to double up on the CI. New versions of Go happen so seldom,
and we change this requirement so infrequently, that I think it's ok
to use fixed versions.

Merge pull request #7 from mromaszewicz/lint

Parameterize Go versions for lint CI

feat(ci)!: remove < Go 1.22

As `oapi-codegen` has migrated to a minimum of Go 1.22.

feat(ci): don't allow auto-downloaded Go toolchain updates

feat(ci): allow excluding Go versions to test against

As noted in #2, some of our projects have a minimum Go version required,
so it should be possible to opt-out.

Co-authored-by: Claude Sonnet 4.5 <github-copilot@jamietanna.co.uk>