chore: fixes compile error for security iap and firebase sample. (#1900)

This fixes the compile error in iap-sample and firebase-sample and should unblock Spring Boot 3.1.0 #1874. 

However, iap sample does not run as described in [README](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/main/spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/README.adoc?plain=1#L52-L64) for me.
firebase sample also not working as expected.

Neither sample come with test and may have been broken before this change. Need to be further investigated.
Filed #1860 to track.

spring-cloud-gcp-samples/spring-cloud-gcp-security-firebase-sample/pom.xml

@@ -13,7 +13,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <java.version>1.8</java.version>
+        <java.version>17</java.version>
     </properties>
 
     <modelVersion>4.0.0</modelVersion>

spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/pom.xml

@@ -17,7 +17,7 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <java.version>1.8</java.version>
+        <java.version>17</java.version>
     </properties>
 
     <!-- The Spring Framework on Google Cloud BOM will manage spring-cloud-gcp version numbers for you. -->

spring-cloud-gcp-samples/spring-cloud-gcp-security-iap-sample/src/main/appengine/app.yaml

@@ -1,4 +1,4 @@
-runtime: java11
+runtime: java17
 instance_class: F1
 
 env_variables: