Explain differences to dependabot

[koppor]

Meanwhile, dependabot also updates the gradle wrapper:

• https://docs.github.com/en/code-security/reference/supply-chain-security/supported-ecosystems-and-repositories#gradle
• Add support for updating gradle wrapper dependabot/dependabot-core#2223 - example: Bump gradle-wrapper from 7.1.1 to 9.3.1 DmitriiShamrikov/mslinks#44

It is unclear, why one should use this action - should be listed in README.md