[WAITING LTS Java 21] feat/java/examples : add examples

[cboudereau]

• remove compatibility code of JDK17
• use switch expression intead of statement
• [requires at least JDK 18] add snippets to api entrypoint Series.java
• update to LTS JDK 21 when github actions and aws will release compatible SDK to LTS 21 (maybe introduce maven toolchains to provide multiple versions of dataseries for 17 and 21 versions)

[codecov]

Codecov Report

Attention: Patch coverage is 81.25000% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 95.58%. Comparing base (8747ff7) to head (9947a54).

Files	Patch %	Lines
...in/java/io/github/cboudereau/dataseries/Union.java	81.25%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #24      +/-   ##
============================================
- Coverage     98.51%   95.58%   -2.93%     
- Complexity       61       63       +2     
============================================
  Files            11        6       -5     
  Lines           875      204     -671     
  Branches         36       36              
============================================
- Hits            862      195     -667     
+ Misses            4        0       -4     
  Partials          9        9              

Flag	Coverage Δ
java	95.58% <81.25%> (+0.04%)	⬆️
rust	?

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