Rename jinja2_filters/get_jinja2_context/register_jinja2_filters to drop the '2' suffix

[bctiemann]

Proposed Changes

• Rename PluginConfig.jinja2_filters → jinja_filters
• Rename PluginConfig.get_jinja2_context() → get_jinja_context()
• Rename register_jinja2_filters() → register_jinja_filters()
• Update the default auto-discovery file convention from jinja2_env.py → jinja_env.py
• Update all call sites, tests, and documentation accordingly

Justification

PR #22363 introduced three public-facing identifiers with a jinja2 prefix:

• PluginConfig.jinja2_filters attribute
• PluginConfig.get_jinja2_context() method
• register_jinja2_filters() helper in netbox.plugins.registration
• Auto-discovery file convention jinja2_env.py / default resource path

These should simply read jinja (no 2) to match the renamed JINJA_FILTERS setting (which dropped its own 2 in #22288) and to align with how all other references to Jinja in the NetBox codebase are spelled.