[docs] stop specifying a JDK version (#8911)

Stale and not adding value. (Destined to get stale again too.)

---

Review the contribution guidelines below:

<details>
  <summary>Contribution guidelines:</summary><br>

- See
  our [contributor guide](../CONTRIBUTING.md) and
  the [Flutter organization contributor guide]([https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md)
  for general expectations for PRs.
- Larger or significant changes should be discussed in an issue before creating a PR.
- Dart contributions to our repos should follow the [Dart style guide](https://dart.dev/guides/language/effective-dart) and use
  `dart format`.
- Java and Kotlin contributions should strive to follow Java and Kotlin best
  practices ([discussion](#8098)).

</details>

Author: pq

CONTRIBUTING.md

@@ -58,7 +58,6 @@ name and contact info to the [AUTHORS](AUTHORS) file.
 ## Environment set-up
 
 1. Install the latest [Java Development Kit](https://www.java.com/en/download/).
-    - The current Java Development Kit version is: **23**.
     - **[Googlers only]** Install Java from go/softwarecenter instead.
 
 2. Set your `JAVA_HOME` directory in the configuration file for your shell environment.