plugin: fix gosec G115 integer overflow warnings (#7799)

Fix integer overflow conversion warnings (G115) by adding appropriate
suppressions where values are provably bounded.

Fixes: https://github.com/coredns/coredns/issues/7793

Changes:
- Updated 56 G115 annotations to use consistent // #nosec G115 format
- Added 2 //nolint:gosec suppressions for conditional expressions
- Removed G115 exclusion from golangci.yml (now explicitly handled per-line)

Suppressions justify why each conversion is safe (e.g., port numbers
are bounded 1-65535, DNS TTL limits, pool lengths, etc.)

Signed-off-by: Azeez Syed <syedazeez337@gmail.com>

plugin/forward/policy.go

@@ -47,7 +47,7 @@ type roundRobin struct {
 func (r *roundRobin) String() string { return "round_robin" }
 
 func (r *roundRobin) List(p []*proxy.Proxy) []*proxy.Proxy {
-	poolLen := uint32(len(p))
+	poolLen := uint32(len(p)) // #nosec G115 -- pool length is small
 	i := atomic.AddUint32(&r.robin, 1) % poolLen
 
 	robin := []*proxy.Proxy{p[i]}