Hide ineffectual MFA checkbox on user form for team admins (#24656)

Also fixes some capitalization/pluralization issues for user management
for team admins

For #24623.

# Checklist for submitter
- [x] Manual QA for all new/changed functionality

frontend/pages/admin/TeamManagementPage/TeamDetailsWrapper/UsersPage/UsersPage.tsx

@@ -413,7 +413,7 @@ const UsersPage = ({ location, router }: ITeamSubnavProps): JSX.Element => {
           defaultSortDirection="asc"
           actionButton={{
             name: isGlobalAdmin ? "add user" : "create user",
-            buttonText: isGlobalAdmin ? "Add users" : "Create users",
+            buttonText: isGlobalAdmin ? "Add users" : "Create user",
             variant: "brand",
             onActionButtonClick: isGlobalAdmin
               ? toggleAddUserModal
@@ -476,7 +476,7 @@ const UsersPage = ({ location, router }: ITeamSubnavProps): JSX.Element => {
           onCancel={toggleCreateUserModal}
           onSubmit={onCreateUserSubmit}
           defaultGlobalRole={null}
-          defaultTeamRole="observer"
+          defaultTeamRole="Observer"
           defaultTeams={[
             { id: currentTeamDetails.id, name: "", role: "observer" },
           ]}

frontend/pages/admin/UserManagementPage/components/UserForm/UserForm.tsx

@@ -362,7 +362,7 @@ const UserForm = ({
             <SelectRoleForm
               currentTeam={currentTeam || formData.teams[0]}
               teams={formData.teams}
-              defaultTeamRole={defaultTeamRole || "observer"}
+              defaultTeamRole={defaultTeamRole || "Observer"}
               onFormChange={onTeamRoleChange}
               isApiOnly={isApiOnly}
             />
@@ -630,6 +630,7 @@ const UserForm = ({
             renderPasswordSection()}
           {(isPremiumTier || isMfaEnabled) &&
             !formData.sso_enabled &&
+            isModifiedByGlobalAdmin &&
             renderTwoFactorAuthenticationOption()}
           {isPremiumTier ? renderPremiumRoleOptions() : renderGlobalRoleForm()}
         </form>