pinned commit for eslint-plugin-mattermost doesn't exist

[itihas]

focalboard/webapp/package.json

Line 133 in a84bbb6

"eslint-plugin-mattermost": "github:mattermost/eslint-plugin-mattermost#23abcf9988f7fa00d26929f11841aab7ccb16b2b",

additionally, the target repo has a blazon saying it's been replaced by mattermost/eslint-plugin. Unclear to me if this is a drop-in replacement, but the pinned version should be updated to a real reference in any case.