snapcraft: use stable-6.0 branch of lxc part (5.21-edge)

[simondeziel]

I quickly glanced at the changes and LXC_DEVEL=0 caught my attention. There are quite a few commits from @mihalicyn we'd lose though, so maybe we should wait for 6.0.2?

[simondeziel]

Built fine:

Generated snap metadata                                                                                                                  
'--enable-manifest' is deprecated, and will be removed in core24.                                                                        
Version has been set to '0+git.db0a3cbf'                                                                                                 
Generated snap manifest                                                                                                                  
Lint warnings:                                                                                                                           
- library: libnvidia-container-go.so.1: unused library 'lib/libnvidia-container-go.so.1.14.6'. (https://snapcraft.io/docs/linters-library)                                                                                                                                         
- library: libatomic.so.1: unused library 'lib/x86_64-linux-gnu/libatomic.so.1.2.0'. (https://snapcraft.io/docs/linters-library)         
- library: libjson-glib-1.0.so.0: unused library 'lib/x86_64-linux-gnu/libjson-glib-1.0.so.0.600.6'. (https://snapcraft.io/docs/linters-library)                                                                                                                                   
- library: libusbredirhost.so.1: unused library 'lib/x86_64-linux-gnu/libusbredirhost.so.1.0.2'. (https://snapcraft.io/docs/linters-library)                                                                                                                                       
- library: libzpool.so.2: unused library 'zfs-0.8/lib/libzpool.so.2.0.0'. (https://snapcraft.io/docs/linters-library)                    
- library: libnvpair.so.3: unused library 'zfs-2.1/lib/libnvpair.so.3.0.0'. (https://snapcraft.io/docs/linters-library)                  
- library: libuutil.so.3: unused library 'zfs-2.1/lib/libuutil.so.3.0.0'. (https://snapcraft.io/docs/linters-library)                    
- library: libzfs.so.4: unused library 'zfs-2.1/lib/libzfs.so.4.1.0'. (https://snapcraft.io/docs/linters-library)                        
- library: libzfs_core.so.3: unused library 'zfs-2.1/lib/libzfs_core.so.3.0.0'. (https://snapcraft.io/docs/linters-library)              
- library: libzfsbootenv.so.1: unused library 'zfs-2.1/lib/libzfsbootenv.so.1.0.0'. (https://snapcraft.io/docs/linters-library)          
- library: libzpool.so.5: unused library 'zfs-2.1/lib/libzpool.so.5.0.0'. (https://snapcraft.io/docs/linters-library)                    
- library: libzfsbootenv.so.1: unused library 'zfs-2.2/lib/libzfsbootenv.so.1.0.0'. (https://snapcraft.io/docs/linters-library)          
- library: libzpool.so.5: unused library 'zfs-2.2/lib/libzpool.so.5.0.0'. (https://snapcraft.io/docs/linters-library)                    
Version has been set to '0+git.db0a3cbf'                                                                                                 
Created snap package lxd_0+git.db0a3cbf_amd64.snap

[tomponline]

I think this is the right think to do, the LTS point releases will use a specific tag any sha anyway.