Build against Memgraph 3.1 headers and meson 1.3

[vkatter-uantwerpen]

A clean checkout does not build against a stock Memgraph 3.1 install. Two
independent reasons, one commit each.

1. meson stops before compiling

meson.build sets c_std=c23, but meson only recognises that value from 1.4,
while the project declares meson_version: '>=1.3.0'. On meson 1.3.x:

ERROR: Unknown C std ['c23']. Possible values are [... 'c2x' ...]

c2x selects the same standard and is accepted across the whole declared meson
range, so the first commit just changes the spelling.

2. Three mgp_* functions do not exist in 3.1.1

src/memgraph/sync_module.c calls mgp_list_append_move,
mgp_map_insert_move and mgp_unordered_map_make_empty. None are declared in
mg_procedure.h on Memgraph 3.1.1, so the module fails to compile.

The second commit adds guarded shims built on the copying variants
(mgp_list_append, mgp_map_insert, mgp_map_make_empty), and has meson
detect the situation with cc.has_header_symbol instead of requiring
-DSYNC_KGRAPH_MGP_COMPAT to be passed by hand:

Header "mg_procedure.h" has symbol "mgp_list_append_move" : NO
Message: Memgraph headers predate mgp_*_move; enabling compat shims.

Against newer headers the symbol is found, the shims compile out entirely, and
nothing changes.

One thing worth your review

mgp_list_append and mgp_map_insert copy the value, so the shims destroy the
source on success in order to preserve the move-semantics contract the call
sites rely on (they already destroy the value themselves on failure). I believe
that matches what the real *_move functions do, but it is my inference about
the intended contract rather than something the headers state — and if it is
wrong it is a leak or a double free rather than a compile error. You wrote the
call sites, so you are better placed to confirm it than I am.

Nothing currently exercises it either: the shims are only reachable on older
Memgraph headers, and the test suite covers src/sync.c rather than the module.

Verified

Fresh clone of this branch, Memgraph 3.1.1 headers, meson 1.3.2, clang 18:

CC=clang meson setup build -Dmemgraph=enabled -Dmemgraph_include_dir=/usr/include/memgraph
ninja -C build

Builds sync.so with no hand-passed flags, and meson test -C build passes.

[gaperez64]

#2 supersedes this and carries the same commits + the syntax-check failures fixed @vkatter-uantwerpen