Move minimal supported base JDK at least to JDK 21

[sgflt]

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Describe the solution you'd like
JDK 11 is EOL. I personally would go to JDK 21, but also JDK 17 would add benefits in form of records. So BaseBar could be transformed into record and 80 % of boiler plate getters, constructors and so on can be eliminated.

Whoever is building a trading bot now, probably choses Spring Boot 3 (because Spring Boot 2 is EOL) which also supports at least JDK 17.

[TheCookieLab]

Agreed, although from a practical standpoint it'll be down to the intersection of expected return and effort required (both in Ta4j and in downstream clients)