Use "true" not "TRUE" in one ICU function call.

This was evidently missed in commit 6337865f, which generally did
s/TRUE/true/ everywhere.  It escaped notice up to now because ICU
versions before ICU 68 provided definitions of "TRUE" and "FALSE"
regardless.  With ICU 68, it fails to compile.

Per report from Condor.  Back-patch to v11 where 6337865f came in.
(I've not tested v10, where this call originated, but I imagine
it's fine since we defined TRUE in c.h back then.)

Discussion: https://postgr.es/m/7a6f3336165bfe3ca66abcda7966f9d0@stz-bg.com

src/backend/commands/collationcmds.c

@@ -365,7 +365,7 @@ get_icu_language_tag(const char *localename)
 	UErrorCode	status;
 
 	status = U_ZERO_ERROR;
-	uloc_toLanguageTag(localename, buf, sizeof(buf), TRUE, &status);
+	uloc_toLanguageTag(localename, buf, sizeof(buf), true, &status);
 	if (U_FAILURE(status))
 		ereport(ERROR,
 				(errmsg("could not convert locale name \"%s\" to language tag: %s",