Gradle: Bump pl.allegro.tech.build.axion-release from 1.18.18 to 1.21.1

Bumps [pl.allegro.tech.build.axion-release](https://github.com/allegro/axion-release-plugin) from 1.18.18 to 1.21.1.
- [Release notes](https://github.com/allegro/axion-release-plugin/releases)
- [Commits](allegro/axion-release-plugin@v1.18.18...v1.21.1)

---
updated-dependencies:
- dependency-name: pl.allegro.tech.build.axion-release
  dependency-version: 1.21.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

build.gradle.kts

@@ -29,7 +29,7 @@ plugins {
   val kotlinVersion = "2.0.21"
   kotlin("jvm") version kotlinVersion
   kotlin("plugin.spring") version kotlinVersion apply false
-  id("pl.allegro.tech.build.axion-release") version "1.18.18"
+  id("pl.allegro.tech.build.axion-release") version "1.21.1"
   id("com.diffplug.spotless") version "8.1.0"
   id("org.springframework.boot") version "3.5.8" apply false
   id("project-report")