Switch from the XML tailoring rules syntax to the basic one.

[KL-7]

It's not urgent, but most likely something will need to be done since they plan to remove tailoring rules in XML format from the CLDR data. See this CLDR ticket for more information.

According to the ticket they plan these changes for CLDR version 24, so when we decide to upgrade to this version, we'll need to update this part of TailoringImporter to extract a list of tailored characters for a specific locale not from XML like this:

<rules>
  <reset>AE</reset>
  <s>ä</s>
  <t>Ä</t>
  <reset>OE</reset>
  <s>ö</s>
  <t>Ö</t>
  <reset>UE</reset>
  <s>ü</s>
  <t>Ü</t>
</rules>

but from something like this:

<basic_rules>
  @æ,Æ;ä,Ä;ę,Ę.ø,Ø;ö,Ö;ő,Ő;œ,Œ
</basic_rules>

As I said, these changes are not required right now, because we use CLDR 21 at the moment and CLDR 24 is nor even released yet (in fact, it's not yet decided what exactly the new syntax for tailoring rules would be), but I'll leave this issue here as a reminder about the upcoming changes.