build: v6.2.0 — disable AVX-512 autovec, fix v6.1.9 SIGILL

v6.1.9 was built on a GitHub Actions EPYC 7763 runner (AVX-512 capable)
and contained 741 vpbroadcastq EVEX instructions in inlined libstdc++
std::string paths. The resulting binary crashed with SIGILL on every
production node: KVM EPYC (DNS2), Ryzen 5 3600 (SAMI-PC), and any
non-x86_64 node.

cmake/AddCompilerFlags.cmake already set -march=x86-64-v2 -mtune=generic
but GCC 11.4 + libstdc++ inlining still autovectorized some paths to
AVX-512. The fix adds an explicit -mno-avx512f -mno-avx512* block
inside CMAKE_X86_64_BASELINE so the build cannot leak AVX-512 regardless
of the build host's capabilities.

Carries forward the v6.1.9 staking-selfheal fix (f69f087) unchanged.
Bump version 6.1.9 -> 6.2.0 to reflect the build-system change.

See references/avx-512-sigill-build-fix.md for the full diagnosis
recipe and the verification steps.

src/clientversion.h

@@ -6,10 +6,10 @@
 //
 
 // These need to be macros, as version.cpp's and triangles-qt.rc's voodoo requires it
-#define CLIENT_VERSION_MAJOR       6
-#define CLIENT_VERSION_MINOR       1
-#define CLIENT_VERSION_REVISION    9
-#define CLIENT_VERSION_BUILD       0
+#define CLIENT_VERSION_MAJOR       6
+#define CLIENT_VERSION_MINOR       2
+#define CLIENT_VERSION_REVISION    0
+#define CLIENT_VERSION_BUILD       0
 
 // Converts the parameter X to a string after macro replacement on X has been performed.
 // Don't merge these into one macro!