fix: add JSON omitempty/omitzero tags to asyncapiv3 types (Go 1.24)

[lerenn]

Adds omitempty/omitzero to the JSON struct tags of the asyncapiv3 types so the
spec types round-trip cleanly when marshaled (only non-required fields are emitted).
Also fixes the Operation.Description tag (was json:"string"), gives the private
dependencies field a json:"-" tag, and adds the SecuritySchemeType constants.

Continues and supersedes #286 by @justenstall (original commit preserved). Fixes #300.

Why the Go bump

Some fields require the newer omitzero option (binding structs, OAuthFlows,
Components, and Operation.Messages, which must distinguish [] from absent).
omitzero only works when the library is built with Go ≥ 1.24 — and the Dagger
CI pipeline compiles everything with pinned images. Without the bump the tags are
silently ignored. So this PR also:

• bumps go.mod (go 1.21 → 1.24, toolchain go1.21.4 → go1.24.0) — this
  raises the minimum Go version for consumers of the library to 1.24;
• bumps the Dagger CI images, which are the actual build/lint compilers:
  golang:1.21.4-alpine → golang:1.24-alpine, golangci-lint v1.62.0 →
  v1.64.8 (last v1, keeps the v1 .golangci.yml format and bundles Go 1.24);
• drops exportloopref (fully inactivated since Go 1.22's loopvar fix) and an
  unused funlen nolint directive surfaced by the newer linter, and rewraps a few
  over-length doc comments.

Verification

• go generate ./... → zero diff (generated code unchanged)
• go build, go vet, gofmt, and the asyncapi/codegen unit tests pass
• golangci-lint v1.64.8 run via the exact CI image → exit 0

🤖 Generated with Claude Code

[lerenn]

Superseded: the Go 1.24 bump needed to make omitzero effective has been pushed directly onto #286, which now carries the full change. Merging from the original PR to preserve @justenstall's authorship.