Feature: #78899 - TCA maxitems optional

Description

The TCA config setting maxitems for type=select and type=group fields is now an optional setting that defaults to a high value (99999) instead of 1 as before.

Impact

Fields that typically relate to multiple relations like the group element and some select elements no longer need maxitems set to some value to enable multiple values.

Example before:

aField => [
    'config' => [
        'type' => `select',
        'renderType' => 'multipleSideBySide',
        'maxitems' => 99999,
    ],
],

Example after:

aField => [
    'config' => [
        'type' => `select',
        'renderType' => 'multipleSideBySide',
    ],
],

This simplifies TCA of those fields and removes a cross dependency between renderType and maxitems.