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/bufsize/bufsize.go

@@ -18,7 +18,7 @@ type Bufsize struct {
 // ServeDNS implements the plugin.Handler interface.
 func (buf Bufsize) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
 	if option := r.IsEdns0(); option != nil && int(option.UDPSize()) > buf.Size {
-		option.SetUDPSize(uint16(buf.Size))
+		option.SetUDPSize(uint16(buf.Size)) // #nosec G115 -- buffer size fits in uint16
 	}
 	return plugin.NextOrFailure(buf.Name(), buf.Next, ctx, w, r)
 }