Fix inconsistently written compressed files. (#20846)

Before https://github.com/minio/minio/pull/20575, files could pick up indices 
from unrelated files if no index was added.

This would result in these files not being consistent across a set.

When loading, search for the compression indicators and check if they 
are within the problematic date range, and clean up any parts that have 
an index but shouldn't.

The test validates that the signature matches the one in files stored without an index.

Bumps xlMetaVersion, so this check doesn't have to be made for future versions.

cmd/xl-storage-format-v2_test.go

@@ -23,6 +23,7 @@ import (
 	"compress/gzip"
 	"context"
 	"encoding/base64"
+	"encoding/binary"
 	"encoding/json"
 	"fmt"
 	"io"
@@ -583,6 +584,28 @@ func Test_xlMetaV2Shallow_Load(t *testing.T) {
 		}
 		// t.Logf("data := %#v\n", data)
 	})
+	// Test compressed index consistency fix
+	t.Run("comp-index", func(t *testing.T) {
+		// This file has a compressed index, due to https://github.com/minio/minio/pull/20575
+		// We ensure it is rewritten without an index.
+		// We compare this against the signature of the files stored without a version.
+		data, err := base64.StdEncoding.DecodeString(`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`)
+		if err != nil {
+			t.Fatal(err)
+		}
+		var xl xlMetaV2
+		err = xl.Load(data)
+		if err != nil {
+			t.Fatal(err)
+		}
+		for _, v := range xl.versions {
+			// Signature should match
+			if binary.BigEndian.Uint32(v.header.Signature[:]) != 0x8e5a6406 {
+				t.Log(v.header.String())
+				t.Fatalf("invalid signature 0x%x", binary.BigEndian.Uint32(v.header.Signature[:]))
+			}
+		}
+	})
 }
 
 func Test_xlMetaV2Shallow_LoadTimeStamp(t *testing.T) {