Bump SixLabors.ImageSharp and SixLabors.ImageSharp.Drawing (#21)

* Bump SixLabors.ImageSharp and SixLabors.ImageSharp.Drawing

Bumps SixLabors.ImageSharp from 3.1.12 to 4.0.0
Bumps SixLabors.ImageSharp.Drawing from 2.1.7 to 3.0.0

---
updated-dependencies:
- dependency-name: SixLabors.ImageSharp
  dependency-version: 4.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
- dependency-name: SixLabors.ImageSharp.Drawing
  dependency-version: 3.0.0
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

* Keep SixLabors on Apache-2.0 versions; ignore major bumps

ImageSharp v4 / ImageSharp.Drawing v3 moved to the commercial "Six Labors
Split License", whose MSBuild target hard-fails the build without a paid
license key (CI failure on RustPlusBot.Features.Map). Revert the version
bump back to the Apache-2.0 3.1.12 / 2.1.7 line and add Dependabot ignore
rules for major updates of both packages so the bump is not re-proposed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: HandyS11 <valentin-clergue@orange.fr>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

.github/dependabot.yml

@@ -9,3 +9,11 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    ignore:
+      # SixLabors moved ImageSharp v4 / ImageSharp.Drawing v3 to the commercial
+      # "Six Labors Split License", which hard-fails the build without a paid
+      # license key. Stay on the Apache-2.0 3.1.x / 2.1.x line. See PR #21.
+      - dependency-name: "SixLabors.ImageSharp"
+        update-types: ["version-update:semver-major"]
+      - dependency-name: "SixLabors.ImageSharp.Drawing"
+        update-types: ["version-update:semver-major"]