[ImgBot] Optimize images

*Total -- 54.65kb -> 44.08kb (19.34%)

/.github/readme/images/slack-dark-128.png -- 6.45kb -> 4.18kb (35.3%)
/.github/readme/images/mesh-layer5.png -- 48.19kb -> 39.90kb (17.21%)

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

Author: ImgBotApp