Upgrade to Guava 30.1, which warns on Java 7

This change can have large impact from two aspects:
1. It calls out a _large_ impact on the _few_ Java 7 users.
2. It may have _small_ impact on the _many_ Android users.

grpc#4671 tracks gRPC's removal of
Java 7 support. We are quite eager to drop Java 7 support as that would
allow using new language features like default methods. Guava is also
dropping Java 7 support and starting in 30.1 it will warn when used on
Java 7. The purpose of the warning is to help discover users that are
negatively impacted by dropping Java 7 before it becomes a bigger
problem.

The Guava logging check was implemented in such a way that there is an
optional class that uses Java 8 bytecode. While the class is optional at
runtime, the Android build system notices when dexing and fails if
Java 8 language featutres are not enabled. We believe this will not be a
problem for most Android users, but they may need to add to their build:

```
android {
    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }
}
```

See also https://github.com/google/guava/releases/tag/v30.1

android-interop-testing/build.gradle

@@ -25,6 +25,10 @@ android {
             }
         }
     }
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 26
 
     defaultConfig {

android/build.gradle

@@ -8,6 +8,10 @@ plugins {
 description = 'gRPC: Android'
 
 android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 29
     defaultConfig {
         consumerProguardFiles "proguard-rules.txt"

build.gradle

@@ -55,7 +55,7 @@ subprojects {
         javaPluginPath = "$rootDir/compiler/build/exe/java_plugin/$protocPluginBaseName$exeSuffix"
 
         nettyVersion = '4.1.52.Final'
-        guavaVersion = '30.0-android'
+        guavaVersion = '30.1-android'
         googleauthVersion = '0.22.2'
         protobufVersion = '3.12.0'
         protocVersion = protobufVersion

examples/android/clientcache/app/build.gradle

@@ -2,6 +2,10 @@ apply plugin: 'com.android.application'
 apply plugin: 'com.google.protobuf'
 
 android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 27
 
     defaultConfig {

examples/android/helloworld/app/build.gradle

@@ -2,6 +2,10 @@ apply plugin: 'com.android.application'
 apply plugin: 'com.google.protobuf'
 
 android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 27
 
     defaultConfig {

examples/android/routeguide/app/build.gradle

@@ -2,6 +2,10 @@ apply plugin: 'com.android.application'
 apply plugin: 'com.google.protobuf'
 
 android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 27
 
     defaultConfig {

examples/android/strictmode/app/build.gradle

@@ -2,6 +2,10 @@ apply plugin: 'com.android.application'
 apply plugin: 'com.google.protobuf'
 
 android {
+    compileOptions {
+        sourceCompatibility JavaVersion.VERSION_1_8
+        targetCompatibility JavaVersion.VERSION_1_8
+    }
     compileSdkVersion 28
 
     defaultConfig {