ci: Temporarily use clang in valgrind tasks

valgrind currently does not work on GCC -O2 compiled executables, which
contain std::optional use, due to an upstream bug. See
https://bugs.kde.org/show_bug.cgi?id=472329

One workaround could be to use -O1. However, that seems brittle, as
variantions of the bug were seen with -O1 as well.

So temporarily use clang in the valgrind CI tasks, because this also
allows to drop a false-positive suppression for:
-DCMAKE_CXX_FLAGS='-Wno-error=array-bounds'

Also, update the comment in contrib/valgrind.supp to mention the
background:

* GCC -O2 wasn't tested with the suppressions file, due to the mentioned
  bug.
* Clang-17 (or later) on aarch64 wasn't tested due to bug
  https://github.com/bitcoin/bitcoin/issues/29635 and the minimum
  supported clang version is clang-17 right now.
* GUI isn't tested, because it requires a debug build, see the prior
  commit.

This means the only tested config right now is the one mentioned in the
suppression file.

test/sanitizer_suppressions/valgrind.supp

@@ -12,8 +12,10 @@
 #       --error-limit=no build/bin/test_bitcoin
 #
 # Note that suppressions may depend on OS and/or library versions.
-# Tested on aarch64 and x86_64 with Ubuntu Noble system libs, using clang-16
-# and GCC, without gui.
+# Tested on Debian Trixie system libs,
+# * using clang (only x86_64, see https://bugs.kde.org/show_bug.cgi?id=485276),
+# * and GCC (only -O1, see https://bugs.kde.org/show_bug.cgi?id=472329),
+# without gui (because it only passes with a DEBUG=1 depends build).
 {
    Suppress leaks on shutdown
    Memcheck:Leak