feat: remove hardcoded versions in chain-upgrade docs

[conorsch]

Back when the guide was managed in the Penumbra protocol repo [0], the version strings in the docs were automatically bumped in line with the source code changes. Since breaking the guide out into a separate repo, we no longer have the convenience of those automatic updates, so the guide versions will necessarily lag behind. Let's remove the hardcoded version strings from crucial reference pages like the chain-upgrade docs, leaning on node operators to choose the correct version for a given upgrade.

[0] https://github.com/penumbra-zone/penumbra

[github-actions[bot]]

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