Unnecessary static synchronization in ToolchainMojo causes contention in multi-module builds

## Summary

`ToolchainMojo` uses a static `LOCK` object for `synchronized` around `toolchainManagerPrivate.storeToolchainToBuildContext()`. The `storeToolchainToBuildContext` method already operates on a per-session build context, making the static synchronization unnecessary and potentially causing contention in multi-module builds.

## Location

`ToolchainMojo.java:29, 67-69`

https://github.com/apache/maven-toolchains-plugin/blob/master/src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java#L29
https://github.com/apache/maven-toolchains-plugin/blob/master/src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java#L67-L69

## Code

```java
private static final Object LOCK = new Object();

// ...

synchronized (LOCK) {
toolchainManagerPrivate.storeToolchainToBuildContext(tc, session);
}
```

## Problem

1. The plugin is marked `threadSafe = true`, meaning Maven may execute it concurrently for different modules/sessions
2. `storeToolchainToBuildContext()` stores data into the build context, which is already per-session and thread-safe
3. The static lock introduces unnecessary contention: one module's toolchain selection blocks another unrelated module's selection
4. This defeats the purpose of `threadSafe = true`

## Impact

In multi-module Maven builds where the toolchains plugin runs in multiple modules (e.g., via inherited plugin configuration), the static lock serializes what could otherwise run in parallel, slowing down builds.

## Suggested Fix

Remove the `synchronized` block entirely, as `storeToolchainToBuildContext` already handles its own concurrency per session.

Start in src/main/java/org/apache/maven/plugins/toolchain/ToolchainMojo.java at the static LOCK declaration around line 29 and the synchronized block around lines 67-69. Confirm that storeToolchainToBuildContext already uses the per-session build context, then remove the unnecessary synchronization and run the Maven project test suite. Done means the lock and synchronized block are gone without changing toolchain selection behavior.

Written by the indexing model from the issue text.