bgpd: release manual vpn label on instance deletion (backport #18121)

[mergify]

When a BGP instance with a manually assigned VPN label is deleted, the label is not released from the Zebra label registry. As a result, reapplying a configuration with the same manual label leads to VPN prefix export failures.

For example, with the following configuration:

router bgp 65000 vrf BLUE
address-family ipv4 unicast
label vpn export

Release zebra label registry on unconfiguration.

Fixes: d162d5f ("bgpd: fix hardset l3vpn label available in mpls pool")

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This is an automatic backport of pull request #18121 done by Mergify.

[mergify]

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