[MRESOLVER-358] Upgrade some dependencies (s4u#284)

Upgrades
* guava to 31.1-jre (was 30.1-jre)
* mockito-core to 4.11.0 (was 4.8.1)
* plexus-utils to 3.5.1 (was 3.5.0)
* maven deps (used in demos only) to 3.8.7 (was 3.8.4) and add a comment to keep in sync with maven build

---

https://issues.apache.org/jira/browse/MRESOLVER-358

maven-resolver-demos/pom.xml

@@ -38,7 +38,8 @@
   </modules>
 
   <properties>
-    <mavenVersion>3.8.4</mavenVersion>
+    <!-- This is somewhat strange, but let's keep this in sync w/ minimalMavenBuildVersion -->
+    <mavenVersion>3.8.7</mavenVersion>
   </properties>
 
   <build>

pom.xml

@@ -90,7 +90,7 @@
     <checkstyle.violation.ignore>None</checkstyle.violation.ignore>
     <sisuVersion>0.3.5</sisuVersion>
     <guiceVersion>5.1.0</guiceVersion>
-    <guavaVersion>30.1-jre</guavaVersion>
+    <guavaVersion>31.1-jre</guavaVersion>
     <guavafailureaccessVersion>1.0.1</guavafailureaccessVersion>
     <slf4jVersion>1.7.36</slf4jVersion>
     <minimalMavenBuildVersion>[3.8.7,)</minimalMavenBuildVersion>
@@ -178,14 +178,14 @@
       <dependency>
         <groupId>org.mockito</groupId>
         <artifactId>mockito-core</artifactId>
-        <version>4.8.1</version>
+        <version>4.11.0</version>
         <scope>test</scope>
       </dependency>
 
       <dependency>
         <groupId>org.codehaus.plexus</groupId>
         <artifactId>plexus-utils</artifactId>
-        <version>3.5.0</version>
+        <version>3.5.1</version>
       </dependency>
 
       <dependency>