Update `emerge-webrsync` to `emaint sync` (#26352)

Gentoo users using git to sync cannot use `emerge-webrsync` command, but it seems like `emaint sync` still works with the webrsync method. Furthermore, running the version with `emerge-webrsync` on my Gentoo computer, which syncs with git, seemed to break syncing until manually fixed (by deleting and reinstalling the gentoo repository)

.github/workflows/bootstrap_testing.yml

@@ -99,7 +99,7 @@ jobs:
                   zypper --non-interactive install sudo git shadow findutils tar  # findutils=xargs
                   ;;
                 *gentoo*)
-                  emerge-webrsync
+                  emaint sync
                   emerge --noreplace --ask=n sudo dev-vcs/git shadow findutils  # findutils=xargs
                   ;;
                 *archlinux*|*cachyos*|*manjaro*)

util/env-bootstrap.sh

@@ -359,7 +359,7 @@ __EOT__
             *gentoo*)
                 echo "It will also install the following system packages using 'emerge':" >&2
                 print_package_manager_deps_and_delay
-                $(nsudo) emerge-webrsync
+                $(nsudo) emaint sync
                 $(nsudo) emerge --noreplace --ask=n $(get_package_manager_deps | tr ' ' '\n') || signal_execution_failure
                 exit_if_execution_failed
                 ;;