chore: upgrade realm version and migrate deprecated config

[volodymyr-rutskyi]

• Upgrade @redocly/realm to 0.102.0
• Remove the theme. prefix from translation keys (removed in Realm 0.102.0)
• Removed the deprecated theme configuration property from redocly.yaml and front matter (backwards compatibility for it will be dropped in 1.0.0 release)
• Renamed @i18n directory to @l10n and i18n config property to l10n (the feature was renamed and the backwards compatibility for the @i18n directory will be dropped in 1.0.0 release)