Raise minimum version to Java 11 (and test on 21 as well)

[robinst]

Bump actions/setup-java to v2 as well, which requires specifying a distribution and use 8 instead of 1.8.

[robinst]

Ok, will have to raise the minimum Java version first, see error on 21:

Error:  Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.10.1:compile (default-compile) on project commonmark-test-util: Compilation failure: Compilation failure: 
Error:  Source option 7 is no longer supported. Use 8 or later.
Error:  Target option 7 is no longer supported. Use 8 or later.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a88ab23) 94.30% compared to head (01fd773) 94.30%.

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Additional details and impacted files

@@            Coverage Diff            @@
##               main     #299   +/-   ##
=========================================
  Coverage     94.30%   94.30%           
  Complexity      227      227           
=========================================
  Files           124      124           
  Lines          3633     3634    +1     
  Branches        549      549           
=========================================
+ Hits           3426     3427    +1     
  Misses          104      104           
  Partials        103      103           

see 1 file with indirect coverage changes

[robinst]

Let's go to Java 11, see #125 (comment)

Reasons:

• Java 8 was released almost 10 years ago
• We need 9+ to build with modules (unless we use a compatibility plugin like ModiTect; but I don't want that as it complicates the build and moves module-info to poms instead of normal files with IDE support, etc)
• We could go to 9 but it doesn't seem like many people use 9 and not 11. 11 is also a LTS version so is better supported.