Update kotlin -> 1.4.20 & java -> 15

buildSrc/src/main/kotlin/be/simplenotes/java-convention.gradle.kts

@@ -16,8 +16,8 @@ group = "be.simplenotes"
 version = "1.0-SNAPSHOT"
 
 java {
-    sourceCompatibility = JavaVersion.VERSION_14
-    targetCompatibility = JavaVersion.VERSION_14
+    sourceCompatibility = JavaVersion.VERSION_15
+    targetCompatibility = JavaVersion.VERSION_15
 }
 
 tasks.withType<JavaCompile> {

buildSrc/src/main/kotlin/be/simplenotes/kotlin-convention.gradle.kts

@@ -8,12 +8,12 @@ plugins {
 
 dependencies {
     implementation(kotlin("stdlib-jdk8"))
-    implementation(platform("org.jetbrains.kotlin:kotlin-bom:1.4.10"))
+    implementation(platform("org.jetbrains.kotlin:kotlin-bom:1.4.20"))
 }
 
 tasks.withType<KotlinCompile> {
     kotlinOptions {
-        jvmTarget = "14"
+        jvmTarget = "15"
         javaParameters = true
         freeCompilerArgs = listOf(
             "-Xinline-classes",