java/jflex: undeprecate and update to 1.8.1

Further changes:

• use a prebuilt jar file to avoid depending on Maven or Bazel
• swith MASTER_SITES to GitHub
• add BSD3CLAUSE license
• rewrap pkg-descr

PR: 245447
Submitted by: Jashank Jeremy
Approved by: maintainer
MFH: 2020Q2