nextcloud-client: 2.3.3 -> 2.5.0 (#50463)

Updates to the latest version of the desktop client available. Tested
the config migration from `nextcloud-client` 2.3.3 with a Nextcloud
14.0.3 instance (hosted using `services.nextcloud`).

Additionally the derivation required the following changes:

* Dropped `Qt5Sql` patch: this has been fixed upstream and isn't needed
  anymore (furthermore their CMake structure has changed and the patch
  wouldn't apply anymore on 2.5.0).

* Moved to a new upstream repository (nextcloud/desktop), kept
  `fetchgit` to properly fetch submodules.

* Added OpenSSL 1.1 integration: `libsync` (the syncing provided by this
  package) requires 1.1, furthermore the linking flags had to be fixed
  manually by passing `NIX_LDFLAGS` to the derivation.

Furthermore I moved the support for a Gnome3 keyring into its own
wrapper to avoid a full rebuild of the package whenever you alter
`withGnomeKeyring` in an override expressions.

It's still possible to enable keyring (now without recompile) like this:

```
nextcloud-client.override { withGnomeKeyring = true; }
```

To override the derivation itself you now have to use
`nextcloud-client-unwrapped`:

```
nextcloud-client-unwrapped.overrideAttrs (old: {
   src = yoursrc;
})
```