[ImgBot] Optimize images

*Total -- 2,754.46kb -> 2,744.26kb (0.37%)

/src/assets/img/mikan-cat.png -- 183.35kb -> 173.58kb (5.33%)
/src/assets/img/mikan-vtube.svg -- 2,571.11kb -> 2,570.68kb (0.02%)

Signed-off-by: ImgBotApp <ImgBotHelp@gmail.com>

Author: ImgBotApp