Fix cross-version upgrade tests.

TestUpgradeXversion knows how to make the main regression database's
references to pg_regress.so be version-independent.  But it doesn't
do that for plperl's database, so that the C function added by
commit b7e3a52a8 is causing cross-version upgrade test failures.
Path of least resistance is to just drop the function at the end
of the new test.

In <= v14, also take the opportunity to clean up the generated
test files.

Security: CVE-2024-10979

src/pl/plperl/GNUmakefile

@@ -124,6 +124,7 @@ submake:
 
 clean distclean maintainer-clean: clean-lib
 	rm -f SPI.c Util.c $(OBJS) perlchunks.h plperl_opmask.h
+	rm -f sql/plperl_env.sql expected/plperl_env.out
 	rm -rf $(pg_regress_clean_files)
 ifeq ($(PORTNAME), win32)
 	rm -f $(perlwithver).def

src/pl/plperl/input/plperl_env.source

@@ -50,3 +50,6 @@ $$
    }
 
 $$ LANGUAGE plperl;
+
+-- clean up to simplify cross-version upgrade testing
+DROP FUNCTION get_environ();

src/pl/plperl/output/plperl_env.source

@@ -47,3 +47,5 @@ $$
 $$ LANGUAGE plperl;
 WARNING:  attempted alteration of $ENV{TEST_PLPERL_ENV_FOO} at line 12.
 NOTICE:  environ unaffected
+-- clean up to simplify cross-version upgrade testing
+DROP FUNCTION get_environ();