Add Gao Xiang as a reviewer

[dmcgowan]

Gao Xiang has been making significant contributions to containerd over the 2.1 and 2.2 releases, implementing the erofs snapshotter and continuing to develop and review related features. He brings valuable kernel expertise to the project and has been a valuable reviewer not just on erofs, but with the new mount manager and on updates to runtime and shims.

New reviewers require a 1/3 vote of committers. From 10 committers, 4 must approve + new reviewer:

• [x] @hsiangkao
• [x] @AkihiroSuda
• [x] @dmcgowan
• [x] @estesp
• [x] @mikebrow
• [x] @fuweid
• [x] @mxpv
• [x] @dims
• [ ] @kzys
• [x] @samuelkarp
• [ ] @kiashok