Fix links to java client documentation

[ChrisMcD1]

I noticed some of these links were not working since the java client has moved their documentation from GitHub to their new docs site.

There are still 2 broken links that I did not fix in this PR because I could not find the corresponding new file.

https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/writing_exporters.md?plain=1#L350

and

https://github.com/prometheus/docs/blob/main/content/docs/instrumenting/writing_exporters.md?plain=1#L387

But I figured it wouldn't hurt to make a PR for the ones that were trivial to fix!

I believe the CONTRIBUTING.md is asking me to ping the maintainers of the java client since the docs relate to their project, so @fstab @dhoard @tomwilkie

[ChrisMcD1]

@RichiH This seems to fall under "everything else", so you seem appropriate to reach out to according to MAINTAINERS.md