Fix compilation warnings and test a wider range of Elixir versions

[wkirschbaum]

Changes:

• charlists changed to sigil syntax to fix warnings for the upcoming 1.17 elixir version
• test all supported versions of Elixir, including 1.11
• set the minimum Elixir version to 1.11

[tosie]

+1 for this. Can the failing be restarted, please? Seems like the runtime could not be set up:

Action fetch /builds/otp/ubuntu-22.04/builds.txt failed for mirror https://builds.hex.pm,/ with Error: Request timeout: /builds/otp/ubuntu-22.04/builds.txt
Error: Could not fetch /builds/otp/ubuntu-22.04/builds.txt from any hex.pm mirror

Source: https://github.com/qcam/saxy/actions/runs/8779914685/job/24577094671?pr=130

[wkirschbaum]

@tosie not from my side it appears, unless I push an empty commit

[tosie]

@qcam Can you take a look at this, please? Can we help in any way?

[qcam]

Thanks for fixing this.