Fix broken links to Gradle docs about BOM

URL: https://docs.gradle.org/current/userguide/managing_transitive_dependencies.html#sec:bom_import
returns HTTP error status 404. This page is present in documentation
for Gradle 5.6.4 [1], but doesn't exist in documentation of Gradle 6.0
and later versions [2]. Documentation file
"userguide/managing_transitive_dependencies.adoc" got renamed to
"dep-man/03-controlling-transitive-dependencies/controlling-transitive-dependencies.adoc"
in commit [3].

Replace links to this page in JUnit documentation with up-to-date URL:
https://docs.gradle.org/current/userguide/platforms.html#sub:bom_import

[1] https://docs.gradle.org/5.6.4/userguide/managing_transitive_dependencies.html#sec:bom_import
[2] https://docs.gradle.org/6.0/userguide/managing_transitive_dependencies.html#sec:bom_import
[3] 0177e18c63e ("Reshuffle dependency management docs", 2019-05-15)
    gradle/gradle@0177e18#diff-7c6cad628253d79a589fdfdee343caf27bc7baf9212f28ba2a8a43a73ced6600

Closes #3414

Author: rybak

documentation/src/docs/asciidoc/user-guide/appendix.adoc

@@ -107,7 +107,7 @@ artifacts are deployed to Sonatype's {snapshot-repo}[snapshots repository] under
 The _Bill of Materials_ POM provided under the following Maven coordinates can be used to
 ease dependency management when referencing multiple of the above artifacts using
 https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Importing_Dependencies[Maven]
-or https://docs.gradle.org/current/userguide/managing_transitive_dependencies.html#sec:bom_import[Gradle].
+or https://docs.gradle.org/current/userguide/platforms.html#sub:bom_import[Gradle].
 
 * *Group ID*: `org.junit`
 * *Artifact ID*: `junit-bom`

junit-bom/README.md

@@ -4,5 +4,5 @@ This module provides a Bill of Materials POM to ease dependency management using
 or [Gradle]. Please refer to the [User Guide] for details.
 
 [Maven]:      https://maven.apache.org/guides/introduction/introduction-to-dependency-mechanism.html#Importing_Dependencies
-[Gradle]:     https://docs.gradle.org/current/userguide/managing_transitive_dependencies.html#sec:bom_import
+[Gradle]:     https://docs.gradle.org/current/userguide/platforms.html#sub:bom_import
 [User Guide]: https://junit.org/junit5/docs/current/user-guide/#dependency-metadata-junit-bom