Make `jetty-util` classes stop using `java.nio.ByteBuffer`

**Jetty version(s)**
13.0.x
subtask of #14865

**Enhancement Description**
Some classes in the `jetty-util` module do make use of `java.nio.ByteBuffer`, like `SearchPattern`, `AbstractTrie`, `CharsetStringBuilder` and `Utf8StringBuilder`.

This is problematic as the plan is to replace `java.nio.ByteBuffer` with jetty-specific `ReadableBuffer` and `WritableBuffer` classes that are meant to live next to the existing `Retainable` and `RetainableByteBuffer` interfaces in `jetty-io`.

So there are two problems to solve:

- once the migration to `ReadableBuffer` / `WritableBuffer` is done, there will be no access to a `java.nio.ByteBuffer` instance anymore
- `jetty-util` cannot depend on `jetty-io`