[ImgBot] Optimize images

*Total -- 297.42kb -> 271.01kb (8.88%)

/pix/default-thumb.png -- 196.17kb -> 169.91kb (13.38%)
/pix/objc.svg -- 1.61kb -> 1.55kb (3.4%)
/pix/jetbrains.svg -- 4.08kb -> 4.02kb (1.32%)
/pix/jetbrains-mono-white.svg -- 3.50kb -> 3.49kb (0.22%)
/pix/jetbrains-mono-black.svg -- 4.23kb -> 4.23kb (0.18%)
/pix/mulle-objc-logo-opaque.svg -- 87.83kb -> 87.80kb (0.04%)

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

Author: ImgBotApp