Bump version to 0.8.1

Fixes for mbj#412 and mbj#415

Merge pull request mbj#409 from viralpraxis/fix-xstr-with-plain-gvar-…

…unparsing

Fix `xstr` with plain `gvar` unparsing

Merge pull request mbj#384 from mbj/prepare-release

Prepare release

Merge pull request mbj#372 from mbj/fix/keyword-pairs

Fix keyword pairs for sends

Remove support for ruby-3.0

Change version to 0.6.13 (mbj#362)

* Change version to 0.6.13

* Change to checkout@v4

Change version to 0.6.11 (mbj#356)

Merge pull request mbj#353 from mbj/fix/alive-mutation

Fix alive mutation in op-asgn dispatch

Merge pull request mbj#349 from mbj/upgrade/parser

Fix {begin,end}less {i,e}range flipflops

Merge pull request mbj#347 from mbj/fix/arithmetic-csend

Fix binary csend