Update to latest refs and target frameworks

[JimBobSquarePants]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

• Updates workflow to use improved caching of LFS artifacts
• Updates target frameowrk to .NET 8
• Updates submodule to include license key requirement.

@stefannikolei
I opened this before I remembered you had already done #459 and #460
Apologies but I'll close your two in favor of this single one as it saves me LFS charges.

[Copilot]

Pull Request Overview

Updates the project to modernize its target frameworks and improve build infrastructure. The changes upgrade from .NET 6/7 to .NET 8/9, bump the library version to 3.0, and enhance the CI/CD workflow with improved LFS caching and broader Ubuntu package support.

• Upgrades target frameworks from .NET 6.0/7.0 to .NET 8.0/9.0 across all projects
• Updates version from 2.0 to 3.0 and removes obsolete package references
• Improves CI workflow with enhanced LFS caching and comprehensive Ubuntu dependency installation

Reviewed Changes

Copilot reviewed 23 out of 23 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tests/SixLabors.Fonts.Tests/SixLabors.Fonts.Tests.csproj	Updates target frameworks to net8.0/net9.0
tests/SixLabors.Fonts.Benchmarks/SixLabors.Fonts.Benchmarks/SixLabors.Fonts.Benchmarks.csproj	Updates target frameworks to net8.0/net9.0
src/UnicodeTrieGenerator/UnicodeTrieGenerator.csproj	Updates target frameworks and adds IsTrimmable property
src/UnicodeTrieGenerator/Generator.cs	Modernizes code with collection expressions and fixes assembly location usage
src/UnicodeTrieGenerator/Generator.UniversalShapingEngine.cs	Modernizes code with collection expressions and compiler directives
src/UnicodeTrieGenerator/Generator.IndicShapingEngine.cs	Modernizes code with collection expressions and compiler directives
src/SixLabors.Fonts/TextLayout.cs	Modernizes code with collection expressions and improved return types
src/SixLabors.Fonts/Tables/Cff/CffOperand.cs	Adds culture-specific formatting for debug output
src/SixLabors.Fonts/Tables/Cff/CffDataDicEntry.cs	Modernizes string building and debug output formatting
src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs	Improves type specificity in method parameters
src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/IndicShaper.cs	Improves type specificity and adds const modifier
src/SixLabors.Fonts/StreamFontMetrics.TrueType.cs	Improves return type specificity
src/SixLabors.Fonts/StreamFontMetrics.Cff.cs	Improves return type specificity
src/SixLabors.Fonts/SixLabors.Fonts.csproj	Updates target frameworks, version, and removes obsolete dependencies
src/SixLabors.Fonts/IO/ZlibInflateStream.cs	Modernizes exception throwing and code comments
src/SixLabors.Fonts/FontCollection.cs	Modernizes code with collection expressions and improved return types
src/SixLabors.Fonts/BigEndianBinaryReader.cs	Adds missing exception documentation
samples/ListFonts/ListFonts.csproj	Updates target frameworks to net8.0/net9.0
samples/DrawWithImageSharp/DrawWithImageSharp.csproj	Updates target frameworks to net8.0/net9.0
.github/workflows/build-and-test.yml	Improves LFS caching and Ubuntu package installation
.gitattributes	Adds additional file types for LFS tracking
.editorconfig	Enforces collection expressions and object initializers

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