Let M421 C select any point

Fixing #21147

Marlin/src/feature/bedlevel/ubl/ubl.h

 #define UBL_OK false
 #define UBL_ERR true
 
-enum MeshPointType : char { INVALID, REAL, SET_IN_BITMAP };
+enum MeshPointType : char { INVALID, REAL, SET_IN_BITMAP, CLOSEST };
 
 // External references
 

Marlin/src/feature/bedlevel/ubl/ubl_G29.cpp

 
   static bool test_func(uint8_t i, uint8_t j, void *data) {
     find_closest_t *d = (find_closest_t*)data;
-    if ( (d->type == (isnan(ubl.z_values[i][j]) ? INVALID : REAL))
+    if (  d->type == CLOSEST || d->type == (isnan(ubl.z_values[i][j]) ? INVALID : REAL)
       || (d->type == SET_IN_BITMAP && !d->done_flags->marked(i, j))
     ) {
       // Found a Mesh Point of the specified type!
     float best_so_far = 99999.99f;
 
     GRID_LOOP(i, j) {
-      if ( (type == (isnan(z_values[i][j]) ? INVALID : REAL))
+      if (  type == CLOSEST || type == (isnan(z_values[i][j]) ? INVALID : REAL)
         || (type == SET_IN_BITMAP && !done_flags->marked(i, j))
       ) {
         // Found a Mesh Point of the specified type!

Marlin/src/gcode/bedlevel/ubl/M421.cpp

              hasZ = parser.seen('Z'),
              hasQ = !hasZ && parser.seen('Q');
 
-  if (hasC) ij = ubl.find_closest_mesh_point_of_type(REAL, current_position);
+  if (hasC) ij = ubl.find_closest_mesh_point_of_type(CLOSEST, current_position);
 
   if (int(hasC) + int(hasI && hasJ) != 1 || !(hasZ || hasQ || hasN))
     SERIAL_ERROR_MSG(STR_ERR_M421_PARAMETERS);