Update prerequisites for OIDC script

[hknutsen]

Even though secrets are only visible to admins using the GitHub web GUI, using the VS Code extension makes it seem like only write access is needed to create/update them.

Verify if this is true, and update documentation of OIDC script if true.

[helenakallekleiv]

Given the support of GitHub variables in the latest version of GitHub CLI, as described in #407, is this issue still relevant or can it be closed?

[hknutsen]

Depends on the access required to set GitHub configuration variables.

[sondresjolyst]

According to the GitHub Actions Secrets API documentation, a token with secrets:write permissions can create/update secrets as part of a GitHub Actions workflow. However, viewing the secrets through the GitHub web GUI requires admin permissions or secrets:read for tokens.

List repository secrets
Create or update a repository secret