Update default API version to 2026-04

[Firer]

Summary

Bumps the SDK's default API version from 2026-01 to 2026-04 (the current monday.com API version), following the established quarterly version-bump convention.

Changes

• lib/constants/index.ts — CURRENT_VERSION: 2026-01 → 2026-04
• package.json — 14.0.0 → 15.0.0
• CHANGELOG.MD — new [15.0.0] entry

Why a major bump

Changing the default API-Version header is a behavior change for consumers that don't pin a version, so it's released as a major — consistent with the prior default-version bumps (11.0.0 → 2025-07, 12.0.0 → 2025-10, 13.0.0 → 2026-01) and with the quarterly update workflow (yarn version --new-version major).

Scope notes

• Generated code left untouched. lib/generated/sdk.ts and @mondaydotcomorg/api-types are produced by monday's SDK generator and, per the repo README, aren't modified in PRs. This mirrors the previous bump (6802d43), which changed only these same three files.
• 2026-04 is additive. Per the changelog, 2026-04 adds capabilities (e.g. object_relations, notetaker.meetings, document version-history queries, new ExternalWidget enum members) with no removed or renamed fields, so existing typed queries are unaffected by the new default.
• No validation change needed. The existing ^\d{4}-(01|04|07|10)$ check already accepts 2026-04; versionOverride: '2026-04' has worked since before this PR.
• setup-api unaffected. It fetches the schema with version=current, which tracks the current version automatically.

Testing

• yarn build ✅
• yarn test ✅ (33/33)
• yarn lint ✅

Existing tests assert the API-Version header via the DEFAULT_VERSION constant rather than a literal, so they continue to pass against the new default.