test: update exception message asserted due to Spring change

spring-projects/spring-framework#29847
(cherry picked from commit 8d7bd2e)

Author: jgiovaresco

gravitee-apim-gateway/gravitee-apim-gateway-handlers/gravitee-apim-gateway-handlers-api/src/test/java/io/gravitee/gateway/handlers/api/context/ApiTemplateVariableProviderTest.java

@@ -113,7 +113,7 @@ void should_throw_when_evaluate_null_api_properties_in_EL() {
                 assertThat(e)
                     .isInstanceOf(ExpressionEvaluationException.class)
                     .hasCauseInstanceOf(SpelEvaluationException.class)
-                    .hasStackTraceContaining("EL1007E: Property or field 'prop2' cannot be found on null");
+                    .hasStackTraceContaining("EL1012E: Cannot index into a null value");
 
                 return true;
             });

gravitee-apim-gateway/gravitee-apim-gateway-handlers/gravitee-apim-gateway-handlers-api/src/test/java/io/gravitee/gateway/reactive/handlers/api/el/ApiTemplateVariableProviderTest.java

@@ -81,7 +81,7 @@ void should_throw_when_evaluate_null_api_properties_in_EL() {
                 assertThat(e)
                     .isInstanceOf(ExpressionEvaluationException.class)
                     .hasCauseInstanceOf(SpelEvaluationException.class)
-                    .hasStackTraceContaining("EL1007E: Property or field 'prop1' cannot be found on null");
+                    .hasStackTraceContaining("EL1012E: Cannot index into a null value");
 
                 return true;
             });