Add pnpm support

What does this MR do?

Adds pnpm support to the builder images:

• Adds a new pnpm snippet (docker/snippets/pnpm) that installs pnpm globally via npm and verifies the installation
• Pins PNPM_VERSION=11.11.0 in docker/VERSIONS
• Includes pnpm in the default snippets list in docker/scripts/snippets.rb
• Updates the AmazonLinux 2 (x86_64 and arm64) Dockerfiles to install pnpm after Node.js, since they use a platform-specific Node installation

Why is this needed?

Prometheus 3.13 requires pnpm to build its web UI assets. The builder images need pnpm available so omnibus-gitlab can upgrade Prometheus to 3.13 (an LTS release supported until July 2027), replacing the current EOL 3.11.2.

Related issues

• omnibus-gitlab#10031 (closed)

Signed-off-by: psingh29 psingh@gitlab.com