[clangd-vscode] NFC; Improve wording in documentation and update VSCode tasks

Summary:
VSCode tasks are updated to the latest supported versions: deprecated
values are removed and replaced by their new counterparts.

Reviewers: hokein

Reviewed By: hokein

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D76595