[8.0.0] container: extend google_container_node_pool name_prefix max length to 31

[c2thorn]

What changed

• Updates expandNodePool in resource_container_node_pool.go.tmpl to use tpgresource.ReducedPrefixedUniqueId(p) (appending 9 characters) instead of id.PrefixedUniqueId(p) (appending 26 characters) when len(name_prefix) > 14.
• Adds a ValidateFunc to "name_prefix" in schemaNodePool capping maximum prefix length at 31 characters (31 + 9 = 40).
• Adds TestAccContainerNodePool_namePrefix_long to verify that name_prefix values longer than 14 characters provision successfully.
• Updates container_node_pool.html.markdown resource documentation and the v8.0.0 Upgrade Guide (version_8_upgrade.html.markdown).

Why

Fixes hashicorp/terraform-provider-google#8261. Previously, any name_prefix longer than 14 characters combined with the standard 26-character UUID suffix exceeded GKE's 40-character limit for NodePool.name, failing at apply time with HTTP 400. Following the precedent of compute instance templates (PR GoogleCloudPlatform#11448), switching to the 9-character shortened suffix for prefixes > 14 extends the maximum allowed name_prefix length from 14 to 31 characters.

Verification

• Feature Acceptance Test: Executed TestAccContainerNodePool_namePrefix_long (len = 18) against live GKE API; PASSED (HTTP 200 OK, generated name length 27 <= 40).
• Version Upgrade Safety: Executed TestAccContainerNodePool_namePrefix (len = 6) with RELEASE_DIFF=true; PASSED with 0 plan diffs (No changes), confirming zero drift or recreation when upgrading existing 7.x resources to 8.0.0.