Open 10.0.0: bump Version, move the baseline and the security table, add the changelog headings #89

9.1.0 is out (v9.1.0, tagged on release/9.x). This opens 10.0.0 on main once, before its fixes (#53, #76, #78) land, so each one can add its changelog line without bumping the version or fighting over the heading.

• Version 9.1.0 → 10.0.0 in Directory.Build.props
• PackageValidationBaselineVersion 9.0.0 → 9.1.0, so package validation checks 10.0.0 against 9.1.0
• SECURITY.md's supported-versions table 9.0.x → 9.1.x, the latest release, which SecurityPolicyConventionTests now ties to the baseline (SECURITY.md names the version being prepared as supported, not the latest release #79)
• a ## [10.0.0] heading in the root changelog and the four package changelogs. Each package changelog says "No change in this package." until a fix adds a line, and that fix removes it.

The release PR sets the heading's date. It also adds the "what can break you" introduction and the docs/migration.md v9.x section that epic #58 asks for.

Done when main builds as 10.0.0 with the conventions and packaging tests green.