Modernize alloc-no-oom-handling test

- The edition should be 2021 to avoid warnings.

  - The `external_crate` feature was removed in commit 45bf1ed1a112
    ("rustc: Allow changing the default allocator").

    Note that commit d620ae10709c ("Auto merge of #84266") removed
    the old test, but the new one introduced passed the `--cfg` like
    in the old one.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

src/test/run-make-fulldeps/alloc-no-oom-handling/Makefile

@@ -1,4 +1,4 @@
 -include ../tools.mk
 
 all:
-	$(RUSTC) --edition=2018 --crate-type=rlib ../../../../library/alloc/src/lib.rs --cfg feature=\"external_crate\" --cfg no_global_oom_handling
+	$(RUSTC) --edition=2021 --crate-type=rlib ../../../../library/alloc/src/lib.rs --cfg no_global_oom_handling