opts/swarmopts: add missing build-tag

This was introduced in 9c10a9c, which added
use of the network.ParsePortRange.All method, which uses an iterator and
requires go1.23;

    opts/swarmopts/port.go:172:18: cannot range over pr.All() (value of func type iter.Seq[network.Port]): requires go1.23 or later (-lang was set to go1.16; check go.mod)

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

opts/swarmopts/port.go

@@ -1,3 +1,6 @@
+// FIXME(thaJeztah): remove once we are a module; the go:build directive prevents go from downgrading language version to go1.16:
+//go:build go1.24
+
 package swarmopts
 
 import (