[Modern UI] Fix select2 search autofocus

[cconard96]

Q	A
Bug fix?	yes
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
Fixed tickets	-

This fixes two issues with the search input focus:

• The search box is no longer automatically focused when the select2 dropdown is opened. This seems to be a known issue but has not been fixed upstream.
  Search not auto focusing in jQuery 3.6.0 select2/select2#5993
  As a workaround, we need to manually set the focus when the dropdown is opened. The user can still use the arrow keys to select an option when the search is focused.
• When the dropdown is in a Bootstrap modal, it is impossible to interact with the search box. The modal seems to change the focus to its close button instead. For this, we need to be able to specify the dropdownParent property during the select2 initialization. I added the ability to manually specify the dropdown parent selector or by default it automatically uses the parent modal or document body (select2 default).

[cedric-anne]

I am not sure giving ability to redefine the dropdownParentSelector will be really usefull.

If the auto case, which seems to be the only one used here, is fixing the issue, then I am not in favor to add this amount of code in order to add an option we are not using.

[cedric-anne]

@cconard96 Do you know any usecase where using a custom parent selector could be useful ?

[cconard96]

@cconard96 Do you know any usecase where using a custom parent selector could be useful ?

Not at this point. I'll remove that option and simplify the code.