Document support for Gradle 5

Closes gh-15358

Author: wilkinsona

spring-boot-project/spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -162,7 +162,7 @@ Advanced configuration options and examples are available in the {spring-boot-ma
 [[build-tool-plugins-gradle-plugin]]
 == Spring Boot Gradle Plugin
 The Spring Boot Gradle Plugin provides Spring Boot support in Gradle, letting you package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by `spring-boot-dependencies`.
-It requires Gradle 4.4 or later.
+It requires Gradle 4.x (4.4 or later) or 5.x.
 Please refer to the plugin's documentation to learn more:
 
 * Reference ({spring-boot-gradle-plugin-docs}[HTML] and {spring-boot-gradle-plugin-pdfdocs}[PDF])

spring-boot-project/spring-boot-docs/src/main/asciidoc/getting-started.adoc

@@ -43,7 +43,7 @@ Explicit build support is provided for the following build tools:
 | 3.3+
 
 | Gradle
-| 4.4+
+| 4.x (4.4 and later) and 5.x
 |===
 
 
@@ -197,7 +197,7 @@ In those cases, see <<using-boot-maven-without-a-parent>> for an alternative sol
 
 [[getting-started-gradle-installation]]
 ==== Gradle Installation
-Spring Boot is compatible with Gradle 4.4 and later.
+Spring Boot is compatible with Gradle 4.x (4.4 and later) and 5.x.
 If you do not already have Gradle installed, you can follow the instructions at https://gradle.org.
 
 Spring Boot dependencies can be declared by using the `org.springframework.boot` `group`.

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/main/asciidoc/index.adoc

@@ -36,8 +36,8 @@ Andy Wilkinson
 
 The Spring Boot Gradle Plugin provides Spring Boot support in https://gradle.org[Gradle].
 It allows you to package executable jar or war archives, run Spring Boot applications, and use the dependency management provided by `spring-boot-dependencies`.
-Spring Boot's Gradle plugin requires Gradle 4.4 or later.
-If you choose to use the newer Kotlin DSL, it requires Gradle 4.10 or later.
+Spring Boot's Gradle plugin requires Gradle 4.x (4.4 or later) or 5.x.
+If you choose to use the newer Kotlin DSL, it requires Gradle 4.10 or 5.x.
 
 In addition to this user guide, {api-documentation}[API documentation] is also available.