doc: state min I2P router versions, recommend recent ones #36829

In doc/i2p.md, the Compatibility section stated that any SAM v3.1 router can be used. Bitcoin Core now relies on a feature that requires recent routers:

• i2cp.leaseSetEncType=6,4 (introduced in #35696)

This is supported by Java I2P ≥ 2.10.0 and i2pd ≥ 2.58.0.

This pull updates the routers list with those minimum versions and adjusts the Compatibility section.

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The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

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External sites

• Code coverage & benchmarks: https://corecheck.dev/bitcoin/bitcoin/pulls/36829
• Ralph review <img src=https://review.fish.foo/assets/bitcoin/bitcoin/36829.png alt="Ralph review status"> https://git.fish.foo/bitcoin/bitcoin/pulls/36829#author-ralph

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