Fix UTC Serialization in PDFDate.serialize() Method

[Shenile]

This pull request fixes a bug in the PDFDate.serialize() method in in fpdf/syntax.py related to the serialization of UTC dates in PDF metadata. Previously, UTC dates were incorrectly serialized to include timezone information, resulting in formats like "Z00'00'" or "Z15:22". The expected format for UTC dates should simply be "Z".

e.g. Fixes #1261

Checklist:

• A unit test is covering the code added/modified by this PR
• This PR is ready to be merged
• In case of a new feature, docstrings have been added, with also some documentation in the docs/ folder
• A mention of the change is present in CHANGELOG.md

[Lucas-C]

Thank you for your contribution @Shenile 👍

There a few minor details preventing this PR to be merged for now:

• test/linearization.pdf should not be added as part of this PR. It is not needed, and currently makes the GitHub Actions pipeline fail.
• the reference PDF file test/image/image_types/image_types_insert_jpg_flatedecode_windows.pdf has significantly grown, which is a bit strange... Do you have all fpdf2 dependencies up-to-date ? Maybe try to run pip install --update . -r test/requirements.txt

[Shenile]

Thank you for your contribution @Shenile 👍

There a few minor details preventing this PR to be merged for now:

• test/linearization.pdf should not be added as part of this PR. It is not needed, and currently makes the GitHub Actions pipeline fail.
• the reference PDF file test/image/image_types/image_types_insert_jpg_flatedecode_windows.pdf has significantly grown, which is a bit strange... Do you have all fpdf2 dependencies up-to-date ? Maybe try to run pip install --update . -r test/requirements.txt

Thank you for your feedback!

I have removed the test/linearization.pdf file from the PR, as it was not needed. Additionally, I have updated the dependencies by running pip install --update . -r test/requirements.txt .

Please let me know if there's anything else I need to address!

[Lucas-C]

A few unit tests are still failing in the GitHub Actions pipeline:

test/image/image_types/test_insert_images.py::test_insert_jpg_jpxdecode FAILED [ 37%]
test/image/image_types/test_insert_images.py::test_insert_jpg_flatedecode FAILED [ 37%]
test/image/image_types/test_insert_images.py::test_insert_png_alpha_dctdecode FAILED [ 38%]
test/layout/test_page_background.py::test_page_background FAILED         [ 43%]

Each time it's due to the /CreationDate

Could you check if those 4 unit tests pass on the your own development environment @Shenile?