CI: migrate workflows to checkout v5

[rejected-l]

Node 24 compatibility: switch all jobs to actions/checkout@v5. Requires runner v2.327.1+. Pure maintenance, behavior unchanged.

Ref: https://github.com/actions/checkout/releases/tag/v5.0.0

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2

[phdargen]

Hi @rejected-l, note that we require signed commits. See here for instructions on setting up commit signing. Once setup, please rebase against main to re-sign all commits