Fix unreadable release notes blockquote in dark theme (#9957)

* Fix unreadable release notes blockquote in dark theme

MarkdownStyleSheet.fromTheme takes the blockquote text style from the
theme but hard codes the fill as Colors.blue.shade100, so the side panel
drew onSurface text on light blue. Release notes that open with a
blockquote, like 2.60.0, were unreadable in the dark theme.

The panel now passes its own blockquote decoration, and widget tests
assert the text clears WCAG AA contrast in both themes.

* Point the release note entry at the pull request