[ImgBot] Optimize images (#6005)

*Total -- 13.23kb -> 10.21kb (22.85%)

/public/img/dingtalk.png -- 7.86kb -> 5.96kb (24.1%)
/public/img/slack.png -- 5.38kb -> 4.25kb (21.02%)

Signed-off-by: ImgBotApp <[email protected]>

Co-authored-by: ImgBotApp <[email protected]>

Author: 2 people