ci: update renovate pnpm to 9.15.6

[alan-agius4]

This is the minimum required version in the Angular CLI repo.

 ERR_PNPM_UNSUPPORTED_ENGINE  Unsupported environment (bad pnpm and/or Node.js version)

Your pnpm version is incompatible with "/tmp/renovate/repos/github/angular/angular-cli".

Expected version: ^9.15.6
Got: 9.15.4

This is happening because the package's manifest has an engines.pnpm field specified.
To fix this issue, install the required pnpm version globally.

To install the latest version of pnpm, run "pnpm i -g pnpm".
To check your pnpm version, run "pnpm -v".

[angular-automatic-lock-bot]

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