Upgrade existing JDT LS installation

[yveszoundi]

Problem

There appears to be no "official" way to upgrade the installed JDT LS. You could delete the installation directory, but it's obviously a hack. Probably also need to shut down (how exactly?) the eglot server and (manually?) start again after deleting.

Potential solution

• Check the currently installed version against the latest known available version
• For each opened buffer associated with eglot-java-mode, ensure that the eglot-server is not running, otherwise stop it: Maybe just disabling eglot-java-mode is good enough
• Install the new jdtls version to a temporary folder
• If the installation succeeds
  • Move the old jdtls installation folder to a temporary folder
  • Move the new jdtls to its new destination
  • Delete the old jdtls installation folder
• For each opened buffer associated with java-mode, ensure that the eglot-server is running, by reassociating the buffer with eglot-java-mode

References

This depends on #15 .