MAINTAINERS: add Shubharanshu Mahapatra (Shubhranshu153) as a REVIEWER

[AkihiroSuda]

Shubharanshu Mahapatra (@Shubhranshu153) has been very actively contributing to the project:
https://github.com/containerd/nerdctl/issues?q=is%3Apr+author%3AShubhranshu153

So I'd like to invite @Shubhranshu153 as a reviewer

Needs explicit LGTM from @Shubhranshu153 and 1/3 of the nerdctl Committers ( $ceil \left( 5 \times \frac{1}{3} \right) = 2$ ), 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:

• @Shubhranshu153

nerdctl Committers:

• @ktock
• @fahedouch
• @Zheaoli
• @djdongjin
• @yankay

Core Committers (non-binding)

• @AkihiroSuda

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

This PR will remain open for 7 days.

[ktock]

LGTM, welcome!

[Shubhranshu153]

LGTM

Thank you and happy to contribute more to the repo.

[fahedouch]

LGTM, welcome!

[djdongjin]

LGTM, welcome!

[yankay]

LGTM, welcome!