encoding/xml: fix reserved namespace check to be case-insensitive

Fixes the check for the reserved namespace prefix
"xml" to be case insensitive, so as to match all variants of:

    (('X'|'x')('M'|'m')('L'|'l'))

as mandated by Section 2.3 of https://www.w3.org/TR/REC-xml/

This is a roll forward of CL 203417, which was rolled back by CL 240179.
We've decided that the roll back was incorrect, and any broken tests
should be fixed.

The original CL 203417 was by Tamás Gulácsi.

Fixes #35151
For #39876

Change-Id: I2e6daa7aeb252531fba0b8a56086613e13059528
Reviewed-on: https://go-review.googlesource.com/c/go/+/264024


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>