Update membership.json for tidb-dashboard repo

[baurine]

• Add mornyx as committer @mornyx
• Change shhdgit from committer to maintainer @shhdgit

[ti-chi-bot[bot]]

[APPROVALNOTIFIER] This PR is NOT APPROVED

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[sre-bot]

All committers have signed the CLA.

[ti-chi-bot]

In response to a cherrypick label: cannot checkout size/XS: error checking out size/XS: exit status 1. output: error: pathspec 'size/XS' did not match any file(s) known to git