Proposal: raise the minimum supported Go version (go 1.16 → go 1.25)

The root `go.mod` currently declares `go 1.16` (March 2021) and CI tests on Go 1.18 — both several years out of upstream support (Go supports the two most recent releases).

The old floor has real costs today:

- Test dependencies are frozen: e.g. testify cannot be bumped past v1.8.x (v1.9+ requires Go ≥ 1.17).
- Contributors cannot use anything added to the language or stdlib in the last ~5 years (`any`, generics, `errors.Join`, `slices`/`maps`, ...).
- CI runs on Go 1.18, so the declared 1.16 floor isn't actually exercised anywhere — it's nominal.

**Proposal:** bump the `go` directive to **1.25** (the previous stable release, still supported) and update CI to test 1.25 + current stable. I'd keep the change minimal: directive bump, CI matrix, and only the dependency updates that the old floor was blocking — no big-bang refactor.

If 1.25 feels too aggressive for downstream users, would a more conservative floor (e.g. 1.21+) be acceptable? Any supported version would already unblock the points above.

Happy to send the PR once there's agreement on the target version.