fix: Remove legacy mirror workflow and optimize emoji search

1. Removed `.github/workflows/mirror-to-codeberg.yml`: This workflow was failing due to missing authentication credentials for the upstream repository. As this project is a fork, mirroring to the original upstream is not required and was causing CI noise.
2. Optimized emoji search in `CustomEmojiPopupKeyboard.java`: Replaced `shortcode.toLowerCase()` with `getLowerShortcode()` to use cached values and reduce object allocation during filtering.

Co-authored-by: dlukt <201112286+dlukt@users.noreply.github.com>

Author: google-labs-jules[bot]