docs: add breaking changes section for developers upgrading to Indigo 2025.2

[simons-plugins]

Developers upgrading from 2025.1 (Python 3.11) to 2025.2 (Python 3.13) hit several breaking changes not covered in the current troubleshooting docs.

Suggested addition — a new subsection in common-issues.md headed Upgrading to Indigo 2025.2 / Python 3.13 so developers on older versions can skip past it:

### Upgrading to Indigo 2025.2 / Python 3.13 *(2025.2 only)*

**telnetlib removed from the Python 3.13 stdlib**
Error: ModuleNotFoundError: No module named 'telnetlib'
Fix: add telnetlib-313-and-up to your requirements.txt

**websockets library v14+ — parameter renamed**
Error: TypeError: connect() got unexpected keyword argument 'extra_headers'
Fix: rename extra_headers= to additional_headers= in websockets.connect()
Note: this is a websockets library change (v14+), not Python 3.13 itself.

**matplotlib — legendHandles removed**
Error: AttributeError: 'Legend' object has no attribute 'legendHandles'
Fix: use legend_handles instead (e.g. ax.legend().legend_handles)

**matplotlib — plot_date deprecated**
Warning: MatplotlibDeprecationWarning: plot_date was deprecated in 3.9
and will be removed in 3.11. Use plot() instead.
Fix: replace ax.plot_date(...) with ax.plot(...)

All four items confirmed by running against Python 3.13.9 / matplotlib 3.10.7 / websockets 15.0.1 on a live 2025.2 installation.

[Mirochill]

Looks like this was already covered by #37. The current docs/plugin-dev/troubleshooting/common-issues.md on main includes the "Upgrading to Indigo 2025.2 / Python 3.13" section with entries for telnetlib-313-and-up, websockets.connect(additional_headers=...), legend_handles, and replacing plot_date() with plot(). This may be ready to close.