Using netstandard2.0 and net60

[mee-ironsoftware]

Note: This PR is intended to demonstrate a solution for separating net60 from netstandard2.0 and using different SixLabors.ImageSharp versions, proving that the approach works correctly.

Title

Update IronDrawing to targets netstandard2.0 and net60 with different SixLabors.ImageSharp versions.

Description

This PR updates IronDrawing to target both netstandard2.0 and net60 frameworks. To accommodate these changes, IronDrawing now uses different versions of SixLabors.ImageSharp for each framework: v2.1.3 for netstandard2.0 and v3.0.0 for net60. Some source code adjustments have been made to ensure compatibility with both versions of SixLabors.ImageSharp.

Fixes #SLACK-MESSAGE

Type of change

Please select the relevant option by placing an 'x' inside the brackets, like this: [x].

• [ ] 🐛 Bug fix (non-breaking change which fixes an issue)
• [ ] ✨ New feature (non-breaking change which adds functionality)
• [ ] 💥 Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] 🏗️ Internal/structural update (non-breaking change that improves code quality, organization, or performance)
• [x] 📚 This change requires a documentation update

How Has This Been Tested?

Run through all the Unit tests locally.

Checklist:

Please run through the checklist as much as possible and mark the items completed by placing an 'x' inside the brackets, like this: [x].

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes
• [x] I have successfully run all unit tests on Windows
• [x] I have successfully run all unit tests on Linux

Additional Context

This PR prepares IronDrawing for supporting both netstandard2.0 and net60 frameworks and ensures compatibility with the respective SixLabors.ImageSharp versions.

[artem-iron]

Can you use this opportunity to also add netstandard 2.1 target also? This is needed to enable better mobile compatibility.

[mee-ironsoftware]

Can you use this opportunity to also add netstandard 2.1 target also? This is needed to enable better mobile compatibility.

Good idea.