nginx: introduce migration for old uci conf template

nginx: introduce migration for old uci conf template

Introduce support for migration of old uci conf template to new version.
Uci conf template are saved in config backup. This cause problem on config
restore as old config template might have compatibility problem with new
nginx implementation.

Add logic to migrate the template script at runtime to correctly align
to latest change from nginx and nginx-util.

Fixes: 65a676ed56fb ("nginx: introduce support for dynamic modules")
Fixes: #20904
Signed-off-by: Christian Marangi <redacted>