[Good First Issue]: Upgrade solo version to v0.72.0 in CI workflow

### 🆕🐥 First-Time Friendly

This issue is especially welcoming for people who are new to contributing to the **Hiero C++ SDK**.

We know that opening your first pull request can feel like a big step. Issues labeled **Good First Issue** are designed to make that experience easier, clearer, and more comfortable.

No prior knowledge of Hiero, Hedera, or distributed ledger technology is required - just a basic familiarity with C++ and Git is more than enough to get started.

> [!IMPORTANT]
> ### 📋 About Good First Issues
>
> Good First Issues are designed to make getting started as smooth and stress-free as possible.
>
> They usually focus on:
> - Small, clearly scoped changes
> - Straightforward updates to existing code or docs
> - Simple refactors or clarity improvements
>
> Other kinds of contributions — like larger features, deeper technical changes, or design-focused work — are just as valuable and often use the beginner, intermediate, or advanced labels.

### 👾 Description of the Task

The CI workflow currently pins Hiero Solo to `v0.71.0`. Version `v0.72.0` is now available and should be used to keep the integration test environment current.

The version is set in a single line of the reusable build workflow:

```
.github/workflows/zxc-build-library.yaml
```

Relevant line (line 201):

```yaml
soloVersion: v0.71.0
```

This is a direct string replacement — no C++ knowledge or SDK architecture understanding is needed.

### 💡 Proposed Approach

Update the `soloVersion` value in `.github/workflows/zxc-build-library.yaml` from `v0.71.0` to `v0.72.0`. No other files need to change.

### 👩‍💻 Implementation Steps

- [ ] Open `.github/workflows/zxc-build-library.yaml`
- [ ] Locate line 201 (search for `soloVersion`)
- [ ] Replace `v0.71.0` with `v0.72.0`
- [ ] Confirm no other occurrences of `v0.71.0` remain in the file
- [ ] Open a pull request with the change

### ✔️ Acceptance Criteria

- [ ] `soloVersion` in `.github/workflows/zxc-build-library.yaml` is set to `v0.72.0`
- [ ] No other files are modified
- [ ] No unrelated changes are introduced

---

### 📋 Step-by-Step Contribution Guide

To help keep contributions consistent and easy to review, we recommend following these steps:

- [ ] Comment `/assign` to request the issue
- [ ] Wait for assignment
- [ ] Fork the repository and create a branch
- [ ] Set up the project using the instructions in `README.md`
- [ ] Make the requested changes
- [ ] Sign each commit using `-s -S`
- [ ] Push your branch and open a pull request

Read [Workflow Guide](https://github.com/hiero-ledger/hiero-sdk-cpp/blob/main/docs/training/workflow.md) for step-by-step workflow guidance.
Read [README.md](https://github.com/hiero-ledger/hiero-sdk-cpp/blob/main/README.md) for setup instructions.

❗ Pull requests **cannot be merged** without `S` and `s` signed commits.
See the [Signing Guide](https://github.com/hiero-ledger/hiero-sdk-cpp/blob/main/docs/training/signing.md).

### 🤔 Additional Information

If you have questions while working on this issue, feel free to ask!

You can reach the community and maintainers here: [Hiero-SDK-C++ Discord](https://discord.com/channels/905194001349627914/1337424839761465364)

Whether you need help finding the right file, understanding existing code, or confirming your approach — we're happy to help.

Open .github/workflows/zxc-build-library.yaml and search for soloVersion, currently set to v0.71.0. Change it to v0.72.0, confirm no other occurrences of v0.71.0 remain in that file, and ensure no other files are modified.

Written by the indexing model from the issue text.