[ImgBot] Optimize images

*Total -- 1,762.35kb -> 1,277.88kb (27.49%)

/src/contributor-guide/img/builds-and-checks.png -- 94.80kb -> 63.70kb (32.81%)
/src/contributor-guide/img/pr-code-review.png -- 71.35kb -> 48.61kb (31.86%)
/src/contributor-guide/img/pr-target-branch.png -- 109.75kb -> 75.04kb (31.62%)
/src/contributor-guide/img/pr-dashboard.png -- 456.65kb -> 313.99kb (31.24%)
/src/contributor-guide/img/approve-changes.png -- 113.34kb -> 81.40kb (28.18%)
/src/contributor-guide/img/repo-and-projects.png -- 169.21kb -> 125.93kb (25.58%)
/src/contributor-guide/img/related-prs.png -- 250.98kb -> 187.55kb (25.27%)
/src/contributor-guide/img/slack-channel.png -- 71.33kb -> 54.28kb (23.91%)
/src/contributor-guide/img/deployment-comments.png -- 82.87kb -> 63.16kb (23.78%)
/src/contributor-guide/img/award-labels.png -- 140.86kb -> 108.64kb (22.87%)
/src/contributor-guide/img/test-coverage-labels.png -- 201.21kb -> 155.58kb (22.68%)

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

Author: ImgBotApp