Upgrade libp2p networking dependencies

[mswilkison]

Upgrade libp2p and related networking dependencies to a reviewed, compatible release set. Validate the upgrade against current client behavior and operator rollout requirements.

Both main and dev, checked September 18, pin:

• github.com/libp2p/go-libp2p v0.38.2
• github.com/libp2p/go-libp2p-pubsub v0.13.0
• github.com/libp2p/go-libp2p-kad-dht v0.29.0
• github.com/quic-go/quic-go v0.48.2

Relationship to existing work:

• Bump Go toolchain to 1.25 to unblock dependabot updates (was #3955) #3991 tracks the broader Go/x-crypto/IPLD dependency sequence. This task makes the root libp2p, pubsub, DHT and transport upgrade explicit.
• build: use Go 1.26.8 and preserve glibc 2.31 compatibility #4312 provides the Go/toolchain prerequisite and intentionally leaves module versions unchanged. Coordinate the implementation baseline and dev-to-main promotion with Dev → Main release tracking #4256.

Acceptance criteria:

• Select and document a compatible libp2p/pubsub/DHT/QUIC/WebTransport/Pion set and its Go requirements; review breaking changes and applicable upstream advisories.
• Update manifests and checksums on the intended release baseline, with the necessary toolchain and branch reconciliation.
• Preserve the intended transport set, peer authentication/admission, connection resource settings and bounded handshake behavior.
• Validate authorized-peer connectivity, rejected/revoked-peer handling, discovery, reconnection, and messaging with regression tests.
• Exercise current signing/DKG integration and mixed-version peer compatibility in a controlled environment; document rollout and rollback constraints.
• Attach passing CI and dependency-review results suitable for public disclosure, and document compatibility limitations.

The task preserves existing signing membership policy and group-size requirements.

[linear]

ENG-773