build: lower CMakePresets.json schema to v3 for older cmake compat (#27)

The preset file declared "version": 6 (cmake >= 3.25), causing
"Unrecognized 'version' field" on cmake releases as common as the
3.22.1 shipped by Ubuntu 22.04 LTS.  None of the v4/v5/v6 schema
features are used here -- the file only relies on
configurePresets/buildPresets/testPresets with cacheVariables,
binaryDir, displayName, output.outputOnFailure, and environment.
v3 (cmake >= 3.21) is the lowest schema where "generator" remains
optional so cmake can pick a default; v2 would have required adding
"generator" to every preset.

cmakeMinimumRequired lowered from 3.25 to 3.21 to match.

Verified locally that all three presets (default, debug, sanitize)
configure, build, and pass ctest --preset default (19/19 tests).

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

CMakePresets.json

@@ -1,6 +1,6 @@
 {
-  "version": 6,
-  "cmakeMinimumRequired": { "major": 3, "minor": 25, "patch": 0 },
+  "version": 3,
+  "cmakeMinimumRequired": { "major": 3, "minor": 21, "patch": 0 },
   "configurePresets": [
     {
       "name": "default",

doc/CHANGES

@@ -15740,3 +15740,12 @@ m4_esyscmd_s([cat VERSION]) so AC_INIT picks it up at autoreconf time
 "autoreconf -f" before tagging a release so the committed configure
 script reflects the new VERSION; see the Releasing section in
 README.md.
+
+May 7 2026 [RK]
+
+build: lowered CMakePresets.json schema from v6 to v3 (and
+cmakeMinimumRequired from 3.25 to 3.21) so users on older CMake
+releases such as the 3.22.1 shipped by Ubuntu 22.04 LTS can use
+"cmake --preset" without hitting "Unrecognized 'version' field".  No
+v4/v5/v6 schema features were in use; v3 covers every field present
+in the file.