MAINTAINERS: promote Chengyu Zhu (ChengyuZhu6) from a REVIEWER to a COMMITTER

[AkihiroSuda]

@ChengyuZhu6 has been a reviewer since Aug 2025 (#4472), and he has been very actively contributing to the project:
https://github.com/containerd/nerdctl/pulls?q=author%3AChengyuzhu6+

So I'd like to promote him to a committer.

Needs explicit LGTM from the candidate and 2/3 of the nerdctl Committers ( $ceil \left( 5 \times \frac{2}{3} \right) = 4$ ), according to https://github.com/containerd/project/blob/main/GOVERNANCE.md :

After a candidate has been informally proposed in the maintainers forum, the
existing maintainers are given seven days to discuss the candidate, raise
objections and show their support. Formal voting takes place on a pull request
that adds the contributor to the MAINTAINERS file. Candidates must be approved
by 2/3 of the current committers by adding their approval or LGTM to the pull
request. The reviewer role has the same process but only requires 1/3 of current
committers.

If a candidate is approved, they will be invited to add their own LGTM or
approval to the pull request to acknowledge their agreement. A committer will
verify the numbers of votes that have been received and the allotted seven days
have passed, then merge the pull request and invite the contributor to the
organization.

For non-core sub-projects, only committers of the repository that the candidate
is proposed for are given votes.

Candidate:

• @ChengyuZhu6

nerdctl Commiters:

• @ktock (nerdctl Committer)
• @fahedouch (nerdctl Committer)
• @Zheaoli (nerdctl Committer)
• @djdongjin (nerdctl Committer)
• @yankay (nerdctl Committer)

Core Committers (non-binding)

• @AkihiroSuda

I'd also like to get a few LGTMs from the Core Committers too. (not necessary)

This PR will remain open for 7 days.

[fahedouch]

LGTM, welcome

[ktock]

LGTM, welcome.

[ChengyuZhu6]

LGTM! Thank you @AkihiroSuda for your trust in my contributions. I'm looking forward to taking on more responsibilities as a committer for nerdctl.

[yankay]

welcome. :-) LGTM

[djdongjin]

LGTM, welcome!