[FLINK-13465][build] Bump javassist to 3.24.0-GA

[zentol]

Bumps javassist to 3.24.0, which added Java 11 support.

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Last check on commit 28b0a2e (Wed Aug 07 08:16:48 UTC 2019)

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CI report:

• 28b0a2e : SUCCESS Build