Use reactor BOM in Framework build [SPR-15162]

[spring-projects-issues]

Brian Clozel opened SPR-15162 and commented

This change will use the gradle dependency management plugin to import the Reactor maven BOM into the Framework build.

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Affects: 5.0 M4

Reference URL: https://github.com/reactor/reactor

Referenced from: commits 17525ef, 63106e7