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/etcd/xfr.go

@@ -8,7 +8,7 @@ import (
 
 // Serial returns the serial number to use.
 func (e *Etcd) Serial(state request.Request) uint32 {
-	return uint32(time.Now().Unix())
+	return uint32(time.Now().Unix()) // #nosec G115 -- Unix time to SOA serial, Year 2106 problem accepted
 }
 
 // MinTTL returns the minimal TTL.