Automatically update GitHub Actions via Dependabot

[sloede]

This workflow checks your other workflows and creates a PR to update your GH actions if a new version for any action you use has been released. We nowadays use this in virtually every repository in the Trixi Framework.

My motivation is that I noticed in several of your workflow files some outdated versions of actions are used, e.g., the checkout action at v1 (instead of the current v3), e.g., here:

t8code/.github/workflows/tests_sc_p4est.yml

Line 59 in fa2f075

- uses: actions/checkout@v1

An example PR that is created by Dependabot can be seen, e.g., here:
trixi-framework/libtrixi#40

All these boxes must be checked by the reviewers before merging the pull request:

As a reviewer please read through all the code lines and make sure that the code is fully understood, bug free, well-documented and well-structured.

General

• The reviewer executed the new code features at least once and checked the results manually

• The code follows the t8code coding guidelines

• New source/header files are properly added to the Makefiles

• The code is well documented

• All function declarations, structs/classes and their members have a proper doxygen documentation

• All new algorithms and data structures are sufficiently optimal in terms of memory and runtime (If this should be merged, but there is still potential for optimization, create a new issue)

Tests

• The code is covered in an existing or new test case using Google Test

Github action

• The code compiles without warning in debugging and release mode, with and without MPI (this should be executed automatically in a github action)

• All tests pass (in various configurations, this should be executed automatically in a github action)

  If the Pull request introduces code that is not covered by the github action (for example coupling with a new library):

  • Should this use case be added to the github action?
  • If not, does the specific use case compile and all tests pass (check manually)

Scripts and Wiki

• If a new directory with source-files is added, it must be covered by the script/find_all_source_files.scp to check the indentation of these files.
• New Datatypes are added to t8indent_custom_datatypes.txt
• If this PR introduces a new feature, it must be covered in an example/tutorial and a Wiki article.

Licence

• The author added a BSD statement to doc/ (or already has one)

[Davknapp]

Thanks for the contribution!