Upgrade to Eclipse 2021-06

[wilkinsona]

[wilkinsona]

With reference to #277 (comment), 2021-06 isn't any better. In that version, it would appear that our dependencies contain a mixture of Java 8 and Java 11 bytecode. This is sufficient to allow our rewriting of DefaultCodeFormatter and NLS to succeed using Java 8-level bytecode. However, a failure still occurs at runtime due to some other dependencies using Java 11-level bytecode:

io/spring/javaformat/org/eclipse/text/edits/TextEdit has been compiled by a more recent version of the Java Runtime (class file version 55.0), this version of the Java Runtime only recognizes class file versions up to 52.0

With the current approach at least, it appears that 2021-03 is the latest version of Eclipse that we can use without requiring command line builds to perform format checks using Java 11.

[philwebb]

We'll try again to jump to 2021-09 in #277