Cannot perform show javadoc with a JDK 1.8 platform

### Apache NetBeans version

Apache NetBeans 15

### What happened

Cannot perform Javadoc on the core java libraries when using JDK 1.8 as the Java Platform to compile.
When switching to a more recent JDK (e.g. 14), no problem.
The URL in the The Java Platform Manager is https://docs.oracle.com/javase/8/docs/api/

### How to reproduce

- Create a new Gradle Project
- Perform Alt+F1 with the cursor on the `println` method : Javadoc is displayed
- Change the project properties to switch Java Platform to 1.8 on the Build/Compile panel
- Perform the same javadoc search : no Javadoc displayed
- Perform the search via Shift+F1 insted : Javadoc is shown

### Did this work correctly in an earlier version?

Apache NetBeans 14

### Operating System

Windows 10 Professionnel (21H2), Intel(R) Core(TM) i7-7700HQ

### JDK

openjdk version "14" 2020-03-17 / OpenJDK Runtime Environment (build 14+36-1461) / OpenJDK 64-Bit Server VM (build 14+36-1461, mixed mode, sharing)

### Apache NetBeans packaging

Apache NetBeans provided installer

### Anything else

Always reproductible

### Are you willing to submit a pull request?

No

### Code of Conduct

Yes

Reproduce the issue with a Gradle project, JDK 1.8 selected, and Alt+F1 on println, then compare it with Shift+F1 and a newer JDK. Trace the Javadoc lookup and Java Platform handling involved in the two actions. Done means Alt+F1 displays the Java 8 core-library Javadoc while JDK 1.8 is selected, with a regression test for the case.

Written by the indexing model from the issue text.