Remove deprecated `FileUtils.getFile()` to fix `%file%` in Tb 122 and later

[RealRaven2000]

The function FileUtils.getFile() is deprecated from Thunderbird beta 122.

SmartTemplates uses it to test for existence of templates files, which can be replaced with IOUtils.exists(). This leads to problems with using %file% from this beta forward.

Patched version below:

smartTemplate-fx-4.4pre15.zip

[RealRaven2000]

Implemented in Version 4.4.3 - 15/04/2024