Revert "MFV: xz 5.6.0"

This commit reverts 8db56defa766eacdbaf89a37f25b11a57fd9787a,
rolling back the vendor import of xz 5.6.0 and restoring the
package to version 5.4.5.

The revert was not directly due to the attack (CVE-2024-3094):
our import process have removed the test cases and build scripts
that would have enabled the attack. However, reverting would
help to reduce potential confusion and false positives from
security scanners that assess risk based solely on version
numbers.

Another commit will follow to restore binary compatibility with
the liblzma 5.6.0 library by making the previously private
symbol (lzma_mt_block_size) public.

PR: 278127
MFC after: 3 days