Since commit 207cf3d5d4 ('libnm: normalize "connection.uuid"') the
"connection.uuid" is normalized to be a valid UUID and all lower case.
That means, if we have .nmmeta files on disk with a previously valid,
but now invalid UUID, the meta file is no longer going to match.
Reject such file outright as invalid. If we really wanted to preserve
backward compatibility, then we would have to also normalize the
filename when we read it. However, that means, that suddenly we might
have any number of compatible .nmmeta files that normalize to the same
UUID, like the files
71088c75dec54119ab41be71bc10e736aaaabbbb.nmmeta
F95D40B4-578A-5E68-8597-39392249442B.nmmeta
f95d40b4-578a-5e68-8597-39392249442b.nmmeta
Having multiple places for the nmmeta file is complicated to handle.
Also, we often have the connection profile (and the normalized UUID)
first, and then check whether it has a .nmmeta file. If we would support
those unnormalized file names, we would have to visit all file names and
try to normalize it, to find those with a matching UUID.
Non-normalized UUIDs really should not be used and they already are not
working anymore for the .nmmeta file. This commit only outright rejects
them. This is a change in behavior, but the behavior change happened
earlier when we started normalizing "connection.uuid".
423e83b880