resolves 60: update workflow for debian:13

[prudhomm]

Add Debian Trixie/13 support to feelpp-env workflow Fixes#60

[Copilot]

Pull Request Overview

This PR adds support for Debian 13 (Trixie) to the feelpp-env workflow and updates GitHub Actions to more recent versions. The changes enable building Docker images for the new Debian release while modernizing the CI pipeline.

• Adds Debian 13 (Trixie) to the build matrix configuration
• Updates GitHub Actions dependencies from v2/v3 to v3/v5/v6
• Cleans up workflow formatting by removing extra blank lines

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
feelpp-env/Dockerfile-debian-13	Creates a symbolic link to the base Debian Dockerfile for Debian 13 support
.github/workflows/feelpp-env.yml	Adds Debian 13 to build matrix and updates GitHub Actions versions

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[Copilot]

This file appears to contain only the text 'Dockerfile-debian' rather than being a proper symbolic link or Dockerfile. If this is intended to be a symbolic link, it should be created using 'ln -s' command, not as a text file containing the target name.

Suggested change

	Dockerfile-debian
	# The contents of Dockerfile-debian should be copied here.
	# For example, if Dockerfile-debian contains:
	# FROM debian:13
	# RUN apt-get update && apt-get install -y ...
	#
	# Then this file should contain the same lines.
	#
	# If you have access to the filesystem, you should delete this file and create a symbolic link:
	# ln -s Dockerfile-debian Dockerfile-debian-13
	#
	# <--- BEGIN: Copy the contents of Dockerfile-debian below this line --->