Not possible to install

[piiskop]

kalmer@ons:~/email-to-pdf-converter$ ./gradlew

Downloading https://services.gradle.org/distributions/gradle-6.6.1-bin.zip
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Welcome to Gradle 6.6.1!

Here are the highlights of this release:

• Experimental build configuration caching
• Built-in conventions for handling credentials
• Java compilation supports --release flag

For more details see https://docs.gradle.org/6.6.1/release-notes.html

Starting a Gradle Daemon (subsequent builds will be faster)

FAILURE: Build failed with an exception.

• Where:
  Build file '/home/kalmer/email-to-pdf-converter/build.gradle'

• What went wrong:
  Could not compile build file '/home/kalmer/email-to-pdf-converter/build.gradle'.

startup failed:
General error during semantic analysis: Unsupported class file major version 63