Support AsciiDoc-style headings ( = instead of #)

```
$ ./gradlew --version

------------------------------------------------------------
Gradle 8.12
------------------------------------------------------------

Build time: 2024-12-20 15:46:53 UTC
Revision: a3cacb207fec727859be9354c1937da2e59004c1

Kotlin: 2.0.21
Groovy: 3.0.22
Ant: Apache Ant(TM) version 1.10.15 compiled on August 25 2024
Launcher JVM: 17.0.13 (Eclipse Adoptium 17.0.13+11)
Daemon JVM: Compatible with Java 17, Eclipse Temurin (from gradle/gradle-daemon-jvm.properties)
OS: Mac OS X 12.7.6 x86_64

$ cat gradle/libs.versions.toml | grep spotlessChangelog
spotlessChangelog = "com.diffplug.spotless-changelog:3.1.2"
$ cat CHANGELOG.adoc
= Changelog

== [Unreleased]
$ ./gradlew changelogCheck
> Task :changelogCheck FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':changelogCheck'.
> CHANGELOG.adoc: Needs to have '## [Unreleased]'
```

---

https://docs.asciidoctor.org/asciidoc/latest/sections/titles-and-levels/#section-level-syntax

> The number of equal signs in the marker represents the section level using a 0-based index

> In addition to the equals sign marker used for defining section titles, Asciidoctor recognizes the hash symbol (#) from Markdown.