Use anchor-lang 0.26

[KirillLykov]

Anchor 0.26 has been released, if openbook migrate on it, it would simplify downstream builds for solana repo solana-labs/solana#28924.
Please, update this issues if/when openbook will start using 0.26

[mschneider]

@riordanp did you change to anchor get merged?

[KirillLykov]

Looks like using still older version of anchor. Some crates use even 18.

program/dex/permissioned/Cargo.toml

Line 7 in c85e56d

anchor-lang = "0.18.2"

I don't know if we consider upgrade too risky or there are other considerations like directly go to openbook-v2