Open 2.3.1: release/2.3.1 from v2.3.0, Version, changelog section #199

Decision 94's patch release: release/2.3.1 is cut from the tag it patches, v2.3.0 (edeb90c), not from main.

• Cut release/2.3.1 from v2.3.0 and push it.
• On a build/open-2-3-1 branch into it: Directory.Build.props Version 2.3.1, and PackageValidationBaselineVersion 2.3.0 (it is 2.2.0 at the tag), so ApiCompat holds 2.3.1 to 2.3.0's public surface.
• CHANGELOG.md: a ## 2.3.1 — Unreleased section with its first line, because pack fails while the section is empty.
• Check that CI runs on a pull request into release/2.3.1 (the workflows' branch filters) and that PR hygiene accepts the 2.3.1 milestone.