Refactor JWT secret generating & decoding code (#29172)

Old code is not consistent for generating & decoding the JWT secrets.

Now, the callers only need to use 2 consistent functions:
NewJwtSecretWithBase64 and DecodeJwtSecretBase64

And remove a non-common function Base64FixedDecode from util.go

modules/util/util_test.go

@@ -4,7 +4,6 @@
 package util
 
 import (
-	"encoding/base64"
 	"regexp"
 	"strings"
 	"testing"
@@ -234,16 +233,3 @@ func TestToPointer(t *testing.T) {
 	val123 := 123
 	assert.False(t, &val123 == ToPointer(val123))
 }
-
-func TestBase64FixedDecode(t *testing.T) {
-	_, err := Base64FixedDecode(base64.RawURLEncoding, []byte("abcd"), 32)
-	assert.ErrorContains(t, err, "invalid base64 decoded length")
-	_, err = Base64FixedDecode(base64.RawURLEncoding, []byte(strings.Repeat("a", 64)), 32)
-	assert.ErrorContains(t, err, "invalid base64 decoded length")
-
-	str32 := strings.Repeat("x", 32)
-	encoded32 := base64.RawURLEncoding.EncodeToString([]byte(str32))
-	decoded32, err := Base64FixedDecode(base64.RawURLEncoding, []byte(encoded32), 32)
-	assert.NoError(t, err)
-	assert.Equal(t, str32, string(decoded32))
-}