Add Dependabot ecosystem support for Kotlin Toolchain - #45765

Why:

Dependabot is adding support for Kotlin Toolchain (by JetBrains, formerly Amper) as a beta ecosystem, and the docs do not mention it yet.

• Implementation: Add Kotlin Toolchain ecosystem support dependabot/dependabot-core#16203
• Tracking issue: Support for Kotlin Toolchain (Amper) dependabot/dependabot-core#16204

What's being changed (if available, include any code snippets, screenshots, or gifs):

• Adds the dependabot-kotlin-toolchain-support feature flag (data/features/).
• Adds a Kotlin Toolchain row and a short "Kotlin Toolchain" section to the supported package managers reusable: YAML value kotlin-toolchain, toolchain 0.11 and later, the manifest files it updates (project.yaml, module.yaml, *.module-template.yaml, libs.versions.toml), the kotlin/kotlin.bat wrapper scripts, and maven-repository private registries.
• Adds Kotlin Toolchain to the package-ecosystem and cooldown tables in the Dependabot options reference.
• Documents enable-beta-ecosystems, listing Kotlin Toolchain as the ecosystem currently in public preview, with a dependabot.yml example. The "Not currently in use." text is kept for versions without the feature flag.
• Adds a Kotlin Toolchain row to the supported versioning tags table. It uses the same Maven-style qualifiers as Gradle.

Check off the following:

• A subject matter expert (SME) has reviewed the technical accuracy of the content in this PR. In most cases, the author can be the SME. Open source contributions may require an SME review from GitHub staff.
• The changes in this PR meet the docs fundamentals that are required for all content.
• All CI checks are passing and the changes look good in the review environment.