Support JDK v26

[donalmurtagh]

Description:
Installing the JDK failed after I upgraded from v25 to v26

Task version:
v5.7.0

Platform:

• Ubuntu
• macOS
• Windows

Runner type:

• Hosted
• Self-hosted

Repro steps:

- name: Setup JDK
  uses: actions/setup-java@v5.7.0
  with:
    java-version-file: '.sdkmanrc'
    cache: gradle

In .sdkmanrc I made this change

# java=25.0.3-tem
java=26-tem

In build.gradle I also upgraded from v25 to v26

java {
    toolchain {
        languageVersion = JavaLanguageVersion.of(26)
    }
}

Expected behavior:
JDK v26 should install successfully

Actual behavior:
JDK v26 installation fails with the following error

3: Task failed with an exception.
-----------
* What went wrong:
Failed to notify build listener.
> Could not resolve all dependencies for configuration ':core:compileClasspath'.
   > Failed to calculate the value of task ':core:compileJava' property 'javaCompiler'.
      > Cannot find a Java installation on your machine (Linux 6.17.0-1020-azure amd64) matching: {languageVersion=26, vendor=any vendor, implementation=vendor-specific, nativeImageCapable=false}. Toolchain download repositories have not been configured.
> There is 1 more failure with an identical cause.