[1.8.x] Support Paper 1.8.8 / Minecraft 1.8.9 clients

## Description

Add and verify full support for legacy Bukkit/Paper servers in the Minecraft 1.8.x range, including Paper 1.8.8 servers joined by Minecraft 1.8.9 clients.

## Current findings

A live test with MCJars Paper 1.8.8 build 445 showed:

- Plugin Portal 3.8.7 loads and enables successfully.
- Basic commands such as `/pp`, `/pp list`, and `/pp version` work.
- Marketplace commands such as `/pp search ViaVersion` fail while initializing `MarketplacePluginCache`.
- The failure is caused by the runtime's older Guava version:

```
NoSuchMethodError:
CacheBuilder.expireAfterWrite(java.time.Duration)
```

The test server was run on Java 21. The plugin currently targets Java 17 bytecode, so Java 8 host support is a separate compatibility decision.

## Proposed work

- Remove or isolate the dependency on the newer Guava `Duration` overload so marketplace/cache features work with Paper 1.8.8.
- Audit runtime dependencies and Bukkit API usage for 1.8.x.
- Decide whether Java 8 server/runtime support is required, and lower the plugin bytecode target only if all dependencies and language features allow it.
- Add a repeatable MCJars/Paper 1.8.8 smoke test covering startup, `/pp` commands, search, and install.

## Acceptance criteria

- Plugin Portal enables without errors on Paper 1.8.8.
- No cache initialization or shutdown exceptions occur.
- Marketplace search and install work on a clean legacy server.
- The supported Java runtime range is documented clearly.

Reproduce the issue on MCJars Paper 1.8.8 build 445 with Java 21, starting at MarketplacePluginCache and the `/pp search` path. Inspect runtime dependencies and Bukkit API usage for 1.8.x, then verify startup, `/pp` commands, search, and install on a clean legacy server. Done means no cache exceptions and documented Java support.

Written by the indexing model from the issue text.