Migrate to io.spring.develocity.conventions plugin

[sjohnr]

Related gh-15021

See also spring-io/develocity-conventions#82 and spring-io/develocity-conventions#89

[sjohnr]

Note: This also solves missing build scans (for example here) after the dependabot update of develocity in gh-15654. gh-15654 was instead reverted for now.