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What does this implement/fix?

This PR updates GHA to the most recent container. This includes two major upgrades affecting the generated FTL builds:

1. Alpine Linux 3.19 -> 3.20.2 (alpine:latest as of today)
   Nothing special in this release, however, v3.19 already stopped receiving bug fixes. v3.20 will be reach end-of-life at 2026-04-01. Individual points relevant for Pi-hole:
   • LLVM 17 -> 18 (only affecting the clang build), GCC stays at 13.2.1
   • Python 3.11 -> 3.12 (only used during building Mbed TLS and for the Pi-hole API CI tests)
   • 64 bit RISC-V is now supported in stable branches (we had use :edge for riscv64 before)
2. Mbed TLS 3.5.0 -> 3.6.1
   Many security fixes, see Update mbedtls from 3.5.0 to 3.6.1 docker-base-images#96

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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1. I have read and understood the contributors guide, as well as this entire template. I understand which branch to base my commits and Pull Requests against.
2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
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