condition: introduce ConditionVersion=/AssertVersion=

condition: introduce ConditionVersion=/AssertVersion=

Add a new condition wich checks against systemd version.
Change condition_test_kernel_version() into a generic condition_test_version()
so most of the code can be reused.

    $ systemctl --version
    systemd 258 (258~devel-g53ca5f6)

    $ systemd-analyze condition 'ConditionVersion=systemd>255'
    test.service: ConditionVersion=>255 succeeded.

    $ systemd-analyze condition 'ConditionVersion=systemd>260'
    test.service: ConditionVersion=>260 failed.

    $ systemd-analyze condition 'ConditionVersion=systemd>=258'
    test.service: ConditionVersion=>=258 succeeded.

    $ systemd-analyze condition 'ConditionVersion=systemd>=257.1'
    test.service: ConditionVersion=>=257.1 succeeded.

    $ uname -r
    6.12.13-200.fc41.aarch64

    $ systemd-analyze condition 'ConditionVersion=kernel > 4.4'
    test.service: ConditionVersion=kernel > 4.4 succeeded.

    $ systemd-analyze condition 'ConditionVersion=kernel > 6.20'
    test.service: ConditionVersion=kernel > 6.20 failed.

    $ systemd-analyze condition 'ConditionVersion=kernel < 9.0'
    test.service: ConditionVersion=kernel < 9.0 succeeded.