Update GitHub Actions workflows to use .NET 9 instead of .NET Core 3.1.301#6

mikegoatly merged 2 commits intomastermikegoatly/html2md:masterfrom

copilot/fix-27202506-060a-42c6-9d54-b584942c8a25mikegoatly/html2md:copilot/fix-27202506-060a-42c6-9d54-b584942c8a25Copy head branch name to clipboard