Migrate to .NET 8 LTS, consolidate Docker infrastructure, and fix GitHub workflows

[Copilot]

This PR migrates all F# projects from .NET Core 3.0/3.1 to .NET 8 LTS, consolidates Docker infrastructure, and updates GitHub workflows to use supported runner images.

F# Projects Migration

Updated all F# projects to target .NET 8:

• src/visualizations/App.fsproj from netcoreapp3.0 to net8.0
• tests/Visualizations.Tests/Visualizations.Tests.fsproj from netcoreapp3.1 to net8.0
• Owid.Analyzer/Owid.Analyzer.fsproj from netcoreapp3.1 to net8.0

Docker Infrastructure Consolidation

Replaced the problematic two-Dockerfile setup with a single consolidated Dockerfile:

Before: Separate base.Dockerfile (for ghcr.io/sledilnik/website-base) + Dockerfile After: Single Dockerfile with multi-stage build

The new Docker setup:

• Uses node:20-alpine3.22 base image with --platform=$BUILDPLATFORM for native build performance
• Manually installs .NET 8 SDK with multi-architecture support (amd64/arm64)
• Eliminates dependency on potentially outdated base images
• Follows Docker best practices with optimized layer caching

GitHub Workflows

Fixed deprecated actions and updated versions:

• Updated all workflows from ubuntu-20.04 to ubuntu-22.04
• Fixed deprecated actions/[email protected] to actions/cache@v4
• Updated Node.js version from 18 to 20.11.1 across all workflows
• Updated actions/setup-node to v4 and other actions to latest versions

DevContainer Updates

Updated development environment to match production:

• Upgraded from .NET Core 3.1 to .NET 8.0
• Updated Node.js from 14.0 to 20.11.1
• Updated F# runtime settings to net8.0

Package Compatibility

Fixed Node.js 20 compatibility issues:

• Added yarn resolution for @achrinza/[email protected]
• Downgraded @vue/cli-plugin-babel to maintain Vue CLI v4 consistency

Benefits

• Security: .NET 8 LTS support until November 2026
• Performance: Native build platform and latest runtime optimizations
• Reliability: Eliminates base image dependency issues
• Multi-arch: Native support for both amd64 and arm64 architectures
• Maintenance: All workflows use currently supported runner images

The docker build . command now works reliably on both amd64 and arm64 architectures without external image dependencies.

Fixes #1364.

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