bgpd: fix version attribute is an int, not a string

bgpd: fix version attribute is an int, not a string

The json display of the version attribute is originally an
integer. It has changed, most probably mistakenly.

> {
>   "vrfId": 7,
>   "vrfName": "vrf1",
>   "tableVersion": 3,
>   "routerId": "192.0.2.1",
>   "defaultLocPrf": 100,
>   "localAS": 65500,
>   "routes": {
>     "172.31.0.1/32": {
>       "prefix": "172.31.0.1/32",
>       "version": "1",		<--- int or string ??

Let us fix it, by using the integer display instead.

Fixes: f9f2d188e398 ("bgpd: fix 'json detail' output structure")
Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>
(cherry picked from commit c5d7815cccb92c192ca6b752843b8b827a607b53)