GitHub actions: use checkout v3

[tautschnig]

This avoids a warning about a deprected node.js version (version 12).

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[thomasspriggs]

It looks like the equivalent change also needs to be done for the Java models library. Is anyone else already looking at this? https://github.com/diffblue/java-models-library/blob/012cdc25d95ef13ea99123683d0db9044254388d/.github/workflows/models_library_pr.yml#L11

[tautschnig]

It looks like the equivalent change also needs to be done for the Java models library. Is anyone else already looking at this? https://github.com/diffblue/java-models-library/blob/012cdc25d95ef13ea99123683d0db9044254388d/.github/workflows/models_library_pr.yml#L11

Can this be taken care of by someone on the Diffblue team?

[thomasspriggs]

It looks like the equivalent change also needs to be done for the Java models library. Is anyone else already looking at this? https://github.com/diffblue/java-models-library/blob/012cdc25d95ef13ea99123683d0db9044254388d/.github/workflows/models_library_pr.yml#L11

Can this be taken care of by someone on the Diffblue team?

Yes I am happy to take care of this. I just didn't want to duplicate anyone else's efforts.