[MM-21979] Fix incorrect placeholder prop for searchable user list

[fmunshi]

Summary

Placeholder is expected to be of type

    placeholder: {
        id: string;
        defaultMessage: string;
        values?: {string: any};
    };

however there was a recent commit changing the type being passed down to a string causing any component referencing SearchableUserList to error on load

Ticket Link

https://mattermost.atlassian.net/browse/MM-21979

[fmunshi]

Requesting @jgilliam17 for review since the original PR (#4253) was also reviewed by them

QA Steps (Copied from the ticket)

• Open manage channel members modal and ensure that it works
• Open manage team members modal and ensure that it works

[fmunshi]

@jgilliam17 not sure why the first build failed but I pushed a new commit and created a new test server for you

[jgilliam17]

@jgilliam17 not sure why the first build failed but I created pushed a new commit and created a new test server for you

Thank you @fm2munsh

[jgilliam17]

Tested, looks good to merge.

• Both manage channel members modal and manage team members modal work as expected.
• Able to search, remove from channel/team, invite/add and make admin.

[mattermod]

Test server destroyed

[sudheerDev]

LGTM

[jwilander]

LGTM