ci: bump actions/checkout from v3 to v4 in local workflows

## Context

Four local workflow files still pin `actions/checkout@v3`, which runs on Node 16 and has been deprecated by GitHub. Other local workflows in this repo (`validate-examples.yml`, `validate-embedded-examples.yml`) already use `actions/checkout@v4` (Node 20), so this is a consistency and hygiene cleanup.

## Files affected

- `.github/workflows/lint.yml`
- `.github/workflows/new-spec-release.yml`
- `.github/workflows/release.yml`
- `.github/workflows/update-spec.yaml`

## Proposed change

Replace `uses: actions/checkout@v3` with `uses: actions/checkout@v4` in each file above. No other change is needed — the action is backward compatible for the way these workflows use it (checkout only, no custom fetch-depth/token logic that differs between v3 and v4).

## Scope note

Workflows carrying the header `# This action is centrally managed in https://github.com/asyncapi/.github/` (automerge*, lint-pr-title, stale-issues-prs, etc.) sync from `asyncapi/.github` and must not be edited in this repo. Only the four files above are local and safe to modify.

## Willing to submit a PR

Yes.

Update the four named local workflows: .github/workflows/lint.yml, new-spec-release.yml, release.yml, and update-spec.yaml. First confirm each uses actions/checkout@v3 and compare with the existing v4 usage in validate-examples.yml and validate-embedded-examples.yml. Done means only the four local pins use v4; centrally managed workflows remain unchanged, and the workflow configuration checks pass.

Written by the indexing model from the issue text.