[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%)

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Co-authored-by: ImgBotApp <ImgBotHelp@gmail.com>