Replace deprecated .warn() method with .warning()

[lewisemm]

• logging.Logger.warn() has been deprecated since Python 3.3 and will not be available in Python 3.13 as per this pull request (https://github.com/python/cpython/pull/105377).

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (14b6d2c) 82.35% compared to head (cd31b1b) 82.35%.

Files	Patch %	Lines
pipeline/templatetags/pipeline.py	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #807 +/- ## ======================================= Coverage 82.35% 82.35% ======================================= Files 33 33 Lines 1139 1139 ======================================= Hits 938 938 Misses 201 201 ```

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