apt: upgrade 2.6.1 -> 3.0.3

apt: upgrade 2.6.1 -> 3.0.3

Changelog:
https://metadata.ftp-master.debian.org/changelogs/main/a/apt/apt_3.0.3_changelog

Dropped patches which are included in this release, or became obsolete:
0001-Fix-compilation-error-with-clang-libc-18.patch - included in this release
0001-Fix-musl-build.patch - included in this release
0001-Raise-cmake_minimum_required-to-3.13-to-avoid-warnin.patch - included in this release
0001-Remove-using-std-binary_function.patch - became obsolete, fixed upstream
0001-aptwebserver.cc-Include-array.patch - became obsolete, fixed upstream
0001-strutl-Add-missing-include-cstdint-gcc-15.patch - included in this release

Added a new patch to avoid compilation error with musl:
error: 'basename' was not declared in this scope; did you mean 'rename'?

Adapted DEPENDS list - gnutls and gcrypt dependencies were dropped in favor of openssl
in version 2.9.19.

Added a new PACKAGECONFIG, 'usrmerge', which displays a gentle warning if the system
isn't a usrmerge system during package installation.

Added new COMMON_ARCH CMake argument - if it is not defined, CMake is trying to
determine the value of this variable by running dpkg, which is usually a futile
endeavour. It is used in config creation, and to print some system info.

Also adapt a self test: the apt-key command has been deprecated since a while,
and in this release it was completely removed. Instead sources.list file
contains the signature data, on a per-repository basis.

Signed-off-by: Gyorgy Sarvari <skandigraun@gmail.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>