Bump Gradle wrapper from 7.2 to 7.3

The new version came with a warning about buildSrc being configured
with different java bytecode versions for the kotlin and java compile
tasks, and while there's no java code in buildSrc the warning in and
of itself is an annoyance.

buildSrc/build.gradle.kts

@@ -52,6 +52,17 @@ fun readProperties(propertiesFile: File) = Properties().apply {
   }
 }
 
+tasks.withType<JavaCompile>().configureEach {
+  options.release.set(8)
+}
+
+tasks.withType<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>().configureEach {
+  kotlinOptions {
+    jvmTarget = "1.8"
+    allWarningsAsErrors = true
+  }
+}
+
 tasks.check {
   dependsOn(checkBuildConfiguration)
 }

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-7.2-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-7.3-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists