[ImgBot] Optimize images (#244)

*Total -- 92.03kb -> 79.13kb (14.01%)

/public/img/default.png -- 6.25kb -> 3.44kb (44.93%)
/.github/assets/koreanbots-ko.png -- 18.13kb -> 15.22kb (16.09%)
/.github/assets/koreanbots-en.png -- 5.21kb -> 4.49kb (13.71%)
/public/emojis/javascript.png -- 26.98kb -> 23.48kb (12.99%)
/public/emojis/python.png -- 34.38kb -> 31.43kb (8.59%)
/public/emojis/discord.svg -- 1.08kb -> 1.08kb (0.09%)

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

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

public/emojis/discord.svg

@@ -1 +1 @@
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