Upgrade step-runner to version 0.24.0

What does this MR do?

Upgrades the step-runner from version 0.22.0 to 0.24.0.

Step runner changes

v0.24.0

• Job variables can be found using ${{ vars.VAR_NAME }} instead of ${{ job.VAR_NAME }}. See 345.
• Fix issue where functions cannot be fetched locally using an expression. See 350.
• The function directory can be found using ${{ func_dir }} instead of ${{ step_dir }}. See 346.
• Produce and sign a step-runner binaries checksum file. See 348.

v0.23.0

• Expressions are evaluated using a new expression language, Moa. See 337.
• Remove warning about the legacy image. See 338.
• Reference Functions using func:, referencing step: as a fallback. See 340.
• Functions expect func.yml, and load step.yml as a fallback. See 339.
• Link to step-runner binaries in the release notes. See 344.

What are the relevant issue numbers?

• Turn the new expression language on by default (step-runner#309 - closed)
• Rename ${{step_dir}} to ${{func_dir}} for GitLa... (step-runner#339 - closed)
• Using step variables (job/environment, inputs) ... (gitlab#582907 - closed)
• Replace the job context with vars (step-runner#304 - closed)