Upgrade to pnpm 11, migrate package manager config

[karlhorky]

notion-backup pins pnpm 10.33.2 through the top-level packageManager field (package.json). pnpm refers to this field as "legacy" and uses devEngines.packageManager for its pnpm version pin.

Upgrade to pnpm 11.26.0 and move the pin to devEngines.packageManager.

Also, pnpm 11 no longer reads pnpm.onlyBuiltDependencies from package.json:

$ pnpm install --no-frozen-lockfile

[WARN] The "pnpm" field in package.json is no longer read by pnpm.
The following keys were ignored: "pnpm.onlyBuiltDependencies".

Move the existing build approvals to allowBuilds in pnpm-workspace.yaml. Add our standard seven-day dependency release delay and remove .npmrc because strictDepBuilds: true is the pnpm 11 default.

• Upgrade pnpm from 10.33.2 to 11.26.0
• Migrate the pnpm pin to devEngines.packageManager
• Move build approvals to pnpm-workspace.yaml
• Add the seven-day dependency release delay
• Remove the redundant strictDepBuilds setting

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