WL#7036: Deprecate and remove mysql_fix_extensions

The mysql_fix_extensions Perl script aims to assist migration of datadir files
from case-insensitive systems (Windows) to case-sensitive systems (Linux) when
the original files were created without regard to the canonical form (all
upper-case file extensions).  

It is unclear  how old a Windows build one would have to be using to create
files using anything other than the canonical form.  The script is also one of
the most simple scripts written (5 lines). 

The proposal is to deprecate the script.

See BUG#69015 Deprecate mysql_fix_extensions