Point `bash-cache` plugin to new `a8c-ci-toolkit` location

[mokagio]

While I was at it, I also updated the version to the latest, 2.13.0 and fixed the indentation.

This was done via:

find . -type f -name "*.yml" -exec sed -i '' 's/automattic\/bash-cache#[0-9.]\{1,\}/automattic\/a8c-ci-toolkit#2.13.0/g' {} +

If CI is green, we're good to merge.

Internal reference: paaHJt-4z0-p2

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.