Optimize QTimeZone::utc()

Instead of using the QTimeZone ctor taking a string representing
UTC which then first needs to be parsed, use the offset-from-UTC
ctor with an offset of 0.

This approach is much faster, compare the following results for
the tst_QTimeZone::utc benchmark as built on my machine with:

    Config: Using QtTest library 6.10.0, Qt 6.10.0 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 14.2.1 20250207), arch unknown

Before:

    358.686871 nsecs per iteration (total: 358,686,513, iterations: 999999)
    896.524312 CPU cycles per iteration, 2,5 GHz (total: 896,523,416, iterations: 999999)
    2,227.000427 instructions per iteration, 2,484 instr/cycle (total: 2,226,998,200, iterations: 999999)
    560.000375 branch instructions per iteration, 1,56 G/sec (total: 559,999,815, iterations: 999999)

After:

    102.004937 nsecs per iteration (total: 102,004,835, iterations: 999999)
    452.836869 CPU cycles per iteration, 4,44 GHz (total: 452,836,417, iterations: 999999)
    1,182.801068 instructions per iteration, 2,612 instr/cycle (total: 1,182,799,886, iterations: 999999)
    267.143901 branch instructions per iteration, 2,62 G/sec (total: 267,143,634, iterations: 999999)

Relative Delta:

            before   after    improvement
    nsecs   358.7    102.0    3.52x
    cycles  896.5    452.8    1.98x
    instr.  2227.0   1182.8   1.88x
    branch  560.0    267.1    2.10x

This is quite a significant change for such a trivial patch.

Pick-to: 6.8
Change-Id: I5872fe8f1158ff9f6bf306ca8b069559cb59beec
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 765193693982abdbc53910ff0d7d1149332ca83e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/time/qtimezone.cpp

@@ -1455,7 +1455,7 @@ QTimeZone QTimeZone::systemTimeZone()
 */
 QTimeZone QTimeZone::utc()
 {
-    return QTimeZone(QTimeZonePrivate::utcQByteArray());
+    return QTimeZone(0);
 }
 
 /*!