Fix FooConvert 2.1.9 stable tag metadata

[foo-bender]

Summary:

• align readme.txt Stable tag with the 2.1.9 plugin header, FOOCONVERT_VERSION constant, package metadata, and changelog already on canonical master
• restore the WordPress Plugin Check release-metadata invariant without changing executable behavior

Evidence:

• canonical default branch is master at base 187d5fb (Prepare FooConvert 2.1.9 release)
• prior PR Fix MariaDB 12.3 conversion metric queries #5 build failed only because Stable tag 2.1.2 did not match its 2.1.8 plugin header
• repository release history shows prior release preparation synchronized Stable tag with the source version

Verification:

• npm run test:ci-package (3/3 pass)
• npm run test:js (18 files, 81 tests pass)
• npm run test:php (pass with private pro submodule initialized; PHP 8.5 deprecation notices remain)
• npm run build (pass; 3 existing webpack performance warnings)
• npm run package:create-zip:free (pass)
• git diff --check (pass)
• independent read-only review (pass; no security or logic findings)

No merge, release, deployment, tag, package publication, or WordPress.org SVN action is included.