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>

core/dnsserver/quic.go

@@ -53,7 +53,7 @@ func (w *DoQWriter) Close() error {
 // AddPrefix adds a 2-byte prefix with the DNS message length.
 func AddPrefix(b []byte) (m []byte) {
 	m = make([]byte, 2+len(b))
-	binary.BigEndian.PutUint16(m, uint16(len(b)))
+	binary.BigEndian.PutUint16(m, uint16(len(b))) // #nosec G115 -- DNS message length fits in uint16
 	copy(m[2:], b)
 
 	return m