Open 10.1.0 on release/10.x #304

Change

Step 1 of #301, before any fix PR, as a PR against release/10.x. 10.1.0 is a minor because it carries the silent fix #300.

1. Directory.Build.props: Version to 10.1.0 and PackageValidationBaselineVersion to 10.0.0, the release just superseded.
2. SECURITY.md: the supported-versions row to 10.0.x, the latest release on NuGet, and the unsupported row to < 10.0. 9.1.x ended at the v10.0.0 tag. The "How the table changes" and "For how long" prose follows, as main does in build: retarget main to .NET 11 for 11.0.0-preview.1 #292.
3. A ## [10.1.0] section in the root CHANGELOG.md and in each package CHANGELOG.md, undated until the commit that gets tagged (CONTRIBUTING.md, "Changelog"). Each package section says "No change in this package." until a fix PR replaces it. No ### Changed answers heading yet: 10.x on the .NET 11 runtime reads an hour of 24: {24:00:00} parses as {00:00:00} and T24:00:00 as the next day #300's PR adds it.

Done when

The PR is merged into release/10.x, SecurityPolicyConventionTests, ChangelogConventionTests and PackagingConventionTests pass, and every fix PR under #296 adds its line under ## [10.1.0] with pr-hygiene.sh seeing a minor.