aptconfiguration: Set default compression level to 6

aptconfiguration: Set default compression level to 6

Since commit 7a68effcb904b4424b54a30e448b6f2560cd1078, the xz
and lzma compressors read the level of compression they shall
use.

A default of -9 is too much for them, this will use 674 MB,
according to the xz manual page. Level -6 on the other hand
only needs 94 MB memory for compression.

This causes autopkgtest failures in the test-compressed-indexes
test, as not enough memory exists to proceed.

Change the other compression levels to 6 as well: The gzip
and bzip2 FileFd backends do not read them, and use their
code's default level which is 6, so do the same for external
methods.