Fix UI build failure by downgrading gulp-imagemin to v6.2.0 (#10134)

Downgrade gulp-imagemin from v9.2.0 to v6.2.0 to resolve ESM/CommonJS
incompatibility. Version 9.x uses ESM with top-level await which cannot
be loaded with require() in the gulp build task, causing build failures
with error: ERR_REQUIRE_ASYNC_MODULE.

Version 6.2.0 uses CommonJS and is compatible with the existing gulp
setup. This reverts the breaking change introduced in commit 142d9f6.

Co-authored-by: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: rosieyohannan