[backport 3.5] server: don't panic in readonly serializable txn

Problem: We pass grpc context down to applier in readonly serializable txn.
This context can be cancelled for example due to timeout.
This will trigger panic inside applyTxn

Solution: Only panic for transactions with write operations

fixes https://github.com/etcd-io/etcd/issues/14110
main PR https://github.com/etcd-io/etcd/pull/14149

Signed-off-by: Bogdan Kanivets <bkanivets@apple.com>

server/mvcc/kvstore_txn.go

@@ -16,6 +16,7 @@ package mvcc
 
 import (
 	"context"
+	"fmt"
 
 	"go.etcd.io/etcd/api/v3/mvccpb"
 	"go.etcd.io/etcd/pkg/v3/traceutil"
@@ -156,7 +157,7 @@ func (tr *storeTxnRead) rangeKeys(ctx context.Context, key, end []byte, curRev i
 	for i, revpair := range revpairs[:len(kvs)] {
 		select {
 		case <-ctx.Done():
-			return nil, ctx.Err()
+			return nil, fmt.Errorf("rangeKeys: context cancelled: %w", ctx.Err())
 		default:
 		}
 		revToBytes(revpair, revBytes)