Open 2.4.0: Version, package-validation baseline 2.3.0, changelog section #178

CONTRIBUTING "Releasing", step 5, after 2.3.0 shipped on 2026-10-29 (tag v2.3.0 at edeb90c):

• Directory.Build.props: Version 2.4.0, PackageValidationBaselineVersion 2.3.0, so ApiCompat compares the public surface with the released 2.3.0.
• CHANGELOG.md: a ## 2.4.0 — Unreleased section with its first line, because pack fails while the section is empty.
• Once that is on main: cut release/2.4.0 from it and push it.