Bump pnpm version to fix vulnerable transitive dependencies

[Ndiritu]

Bumps package manager to fix vulnerable transitive dependencies that are still being flagged.

Version 8.15.9:

• removes the vulnerable ip package (fix introduced in 8.15.3)
• Upgrades tar to 6.2.1 (fix introduced in 8.15.8

Testing:
rush update --purge
rush rebuild
rush test
.\tools\GenerateModules.ps1 -ModuleToGenerate Users -ApiVersion v1.0 -Build -ExcludeExampleTemplates -ExcludeNotesSection successfully generates the Users module locally.