net: lwm2m: Fix enabler version reporting during bootstrap discovery

LwM2M 1.1 specification mentions that the enabler version format was
specified wrongly in LwM2M 1.0 specification, and servers only "may"
accept the old format, which no longer seems to be the case for Leshan.

The URI reference ("</>;") before the enabler version is now mandatory,
if it's missing the bootstrap discovery fails with Leshan. Another
problem are quotes around the enabler version, which seems to be
conditionally accepted only for LwM2M version 1.0. Therefore, keep the
quotes only for that version, to prevent any potential issues with other
servers.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>