remove included javax.json

[markkampe]

JDK 14 finally includes javax.json, and so complains about my conflicting version.
This is not yet causing trouble for the default (11) JDK, but will soon. When we upgrade to a newer build environment

• update Workspace JRE >=14
• update Project JRE >= 14
• remove javax.json from project build path
• remove javax.json.jar from /lib
• add explanatory note to /lib/README.md (in case someone needs to build on an older JRE)