all: rename tag and flags for legacy support

Rename build tag "proto1_legacy" -> "protolegacy"
to be consistent with the "protoreflect" tag.

Rename flag constant "Proto1Legacy" -> "ProtoLegacy" since
it covers more than simply proto1 legacy features.
For example, it covers alpha-features of proto3 that
were eventually removed from the final proto3 release.

Change-Id: I0f4fcbadd4b5a61c87645e2e5be11d187e59157c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/189345
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/strs/strings.go b/internal/strs/strings.go
index af5f197..2208ff2 100644
--- a/internal/strs/strings.go
+++ b/internal/strs/strings.go
@@ -15,7 +15,7 @@
 
 // EnforceUTF8 reports whether to enforce strict UTF-8 validation.
 func EnforceUTF8(fd protoreflect.FieldDescriptor) bool {
-	if flags.Proto1Legacy {
+	if flags.ProtoLegacy {
 		if fd, ok := fd.(interface{ EnforceUTF8() bool }); ok {
 			return fd.EnforceUTF8()
 		}