Update dependency lerna to v7

[pichlermarc]

The "bootstrap" command was removed by default in v7, and is no longer maintained., which means we'd have to change the way we link packages, possibly by using npm workspaces.

See also:

• PR created by renovate chore(deps): update dependency lerna to v7 - autoclosed #3881
• https://lerna.js.org/docs/legacy-package-management

[pichlermarc]

I applied the lerna bootstrap polyfill suggested in https://lerna.js.org/docs/legacy-package-management#temporarily-polyfilling-legacy-package-management-commands to fix #3925, which also takes care of this issue. However, we should still migrate to npm-workspaces as this is only a temporary solution.

I'll open a follow up-issue.