Attempt to fix missing class error by not using tree artifacts for bootclasspath (#340)

A few times a year across many thousands of builds we encounter a rare error about the `DumpPlatformClassPath` class being missing. Our Bazel setup uses dynamic execution, builds without the bytes, remote execution + remote caching, and path mapping.

The error we encounter is as follows:
```
Error: Could not find or load main class DumpPlatformClassPath
Caused by: java.lang.ClassNotFoundException: DumpPlatformClassPath
```

I'm guessing that this is happening due to some kind of Bazel bug that happens with our Bazel setup and tree artifacts, i.e., declare_directory.

Best I can tell this is happening because the `DumpPlatformClassPath.class` file is somehow not materializing correctly. I'm not 100% confident about that, but it's my leading hypothesis at this point in time.

This commit changes the actions in `bootclasspath.bzl` to not rely on tree artifacts. Instead, they rely JDK 11+'s ability to launch single-file programs (introduced in JEP 330). This avoids the `javac` action and `declare_directory` previously required to compile `DumpPlatformClassPath`.

Problem is this makes rules_java not compatible with JDK's older than 11.

I'm very open to alternative solutions to this, but I haven't yet come up with a robust, cross platform solution that avoids tree artifacts while also maintaining compatibility with JDKs older than 11.

I wanted to open this PR to get some discussion going.

Closes #340

COPYBARA_INTEGRATE_REVIEW=https://github.com/bazelbuild/rules_java/pull/340 from lucidsoftware:bootclasspath-avoid-tree-artifact 34809e22c0022d4bb72088c0b1cfc864ea0cfb31
PiperOrigin-RevId: 880851895
Change-Id: I04c7142981a22b9f4995f25e2789ac6aa2aaf097