Pad Markdown table cells by display width; raise Java baseline to 21

[bwaldvogel]

MarkdownTable padded cells by String.length(). Wide characters (CJK, emoji such as ✅/❌)
take two columns in a monospace font, so the column borders of rows containing them were
shifted. Cells containing | or line breaks broke the table entirely.

• Pad cells by display width: East Asian wide characters and emoji count as two columns,
  combining marks and format characters (e.g. zero width joiner) as none
• Escape | as \| and line breaks as <br>
• The first commit adds tests whose validation files show the broken output, so the fix
  commit shows the difference directly

The display width relies on Character.isEmojiPresentation, which needs Java 21.
Therefore:

• Raise the Java baseline from 17 to 21; CI runs the tests on Java 21, 25 and 27
• Update all dependencies (including JUnit 6 for the tests; the extension is still compiled
  against the JUnit 5 API), Gradle 9.8.1 and the GitHub Actions

[rpost]

lgtm!