2.7.2rc1: Feature improvements

What's Changed

• [2.7] Address client-side vulneralbility for tree-based horizontal XGB by @holgerroth in #3896
• [2.7] Allow one project admin by @IsaacYangSLA in #3897
• [2.7] Cherry pick fixes from main by @YuanTingHsieh in #3910
• [2.7] Change default scheme to HTTP by @nvidianz in #3909
• [2.7] Downloader Rework Backport by @nvidianz in #3908
• [2.7] Remove overseer codes by @IsaacYangSLA in #3899

Full Changelog: 2.7.1...2.7.2rc1