doc: Bump version 8 > 9

[ryanofsky]

Increase version number after bitcoin core update bitcoin/bitcoin#34422. Also update version history and documentation.

[DrahtBot]

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Possible typos and grammar issues:

• "requiring clients to explicitly" -> "requiring clients to explicitly include overloads needed for C++ ↔️ Cap'n Proto type conversions." [the added line is an incomplete sentence fragment and lacks the continuation that makes its meaning clear]
• "installing cmake package files so clients do not" -> "installing cmake package files so clients do not need to use pkgconfig." [the added line is an incomplete sentence fragment and omits the final clause that completes the thought]
• "now requiring include and" -> "now requiring include and includeTypes annotations." [the added line is an incomplete sentence fragment and misses the second item completing the requirement]

^{2026-03-02 00:25:30}

[ryanofsky]

Updated 70ebe77 -> 060464c (pr/v9.1 -> pr/v9.2, compare) adding v7.0 note about introducing version.h

Updated 060464c -> 90bc323 (pr/v9.2 -> pr/v9.3, compare) with more tweaks

Updated 90bc323 -> afa3c91 (pr/v9.3 -> pr/v9.4, compare) with more tweaks

Updated afa3c91 -> b1638ac (pr/v9.4 -> pr/v9.5, compare) with more tweaks