release-74-2
ICU-21508 Rename 'master' to 'main' in docs, maven files, scripts, etc
ICU-22247 remove historical mention of old versions
ICU-21580 change site.icu-project.org to icu.unicode.org etc
ICU-21688 escape angle bracket in non-HTML content
ICU-13702 allow empty string in UnicodeSet
ICU-20088 User Guide migration: Add navigation structure and TOC for Jekyll, fix formatting issues, etc.