Remove my_casedn_str() from get_canonical_filename()

Remove my_casedn_str() from get_canonical_filename()

Under terms of MDEV-31531 we're removing all my_casedn_str() calls, because MDEV-27490 will switch the Unicode version to 14.0.0, and this version does not support inplace lower-casing: some characters can grow in octet length when converting to lower case.

This task is to remove my_casedn_str() calls in get_canonical_filename().

Let's change the code to use CharBuffer so instead of

• Copy from the source to the destination
• Convert the destination to lower-case inplace
  the new code will use
• Copy from the source to the destination with lower-case conversion