Bump min Python from 3.11 to 3.12

[JerryChen97]

Context:
PennyLane is on the way to drop 3.11 (see this and this. Before that happens, Lightning needs to bump to 3.12 first

Description of the Change:

• Bump PYTHON3_MIN_VERSION from "11" to "12" in the wheel build matrix.
• Remove all cp311-* entries from wheel CI workflows (Linux x86_64, Linux aarch64, Linux x86_64 CUDA, Linux aarch64 CUDA, Linux x86_64 ROCm, macOS arm64, Windows x86_64).
• Update upload-nightly-rc-release.yml to use Python 3.12.
• Set requires-python = ">=3.12" in pyproject.toml and remove the Programming Language :: Python :: 3.11 classifier.
• Update version references in setup.py, README.md, and doc/index.rst.

Benefits:
Keeps PennyLane-Lightning in sync with the PennyLane core Python version policy, reducing CI build time and maintenance surface.

Possible Drawbacks:
Users still on Python 3.11 will need to upgrade to 3.12+.

Related GitHub Issues:
[sc-123549]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.28%. Comparing base (43332cf) to head (dda13d7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1395      +/-   ##
==========================================
+ Coverage   96.21%   96.28%   +0.06%     
==========================================
  Files         278      329      +51     
  Lines       40023    47017    +6994     
==========================================
+ Hits        38508    45269    +6761     
- Misses       1515     1748     +233     

Flag	Coverage Δ
unit_tests	96.28% <100.00%> (+0.06%)	⬆️

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[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[maliasadi]

Thanks @JerryChen97!

Pls also update:

• changelog
• setup.py
• readthedocs.yml
• README

[JerryChen97]

Thanks @JerryChen97!

Pls also update:

• changelog
• setup.py
• readthedocs.yml
• README

.readthedocs.yml has already been using 3.12

[JerryChen97]

Also: Do we want to bump to 3.12 in pyproject.toml as well in this PR @maliasadi

[josephleekl]

can. you update this too

pennylane-lightning/doc/index.rst

Line 15 in faa8e83

PennyLane supports Python 3.11 and above.

[JerryChen97]

can. you update this too

pennylane-lightning/doc/index.rst

Line 15 in faa8e83

PennyLane supports Python 3.11 and above.

@josephleekl Done!

[josephleekl]

thanks @JerryChen97 !

[JerryChen97]

In case it breaks Catalyst CI drastically and unnecessarily, I'll hold until Catalyst's bump is merged