strongswan: rename 'local_sourceip' to 'vips'

strongswan: rename 'local_sourceip' to 'vips'

The name 'local_sourceip' was misleading, since the option actually
refers to virtual IP addresses (VIPs) used for routing/binding,
not just a single local source IP. Renaming it to 'vips' better
reflects its purpose and makes the configuration more intuitive
for users, especially in setups with multiple virtual IPs.

Add a uci-defaults migration script to convert existing 'local_sourceip'
entries (whether stored as a plain option or as a list) to a
'vips' list on upgrade.

Signed-off-by: Florian Eckert <redacted>