Update spring-boot 2.3

Update spring-boot 2.3

- update spring boot (2.3.7 -> 2.3.12)
- remove version pinning for grpc in common pom [0]
- remove version pinning for spring-cloud-sleuth in common pom [1]
- replace spring-data-hal-browser with -explorer since the former
  is deprecated

[0] this effectively upgrades some of the grpc deps that have been
    overwritten (1.25.0 -> 1.30.2)
[1] this effectively upgrades sleuth (2.0.2 -> 2.2.5)

Issue-ID: SO-4176
Change-Id: Ibb7d9c3f8938e48bcde8be6cb9ff54f8037915a8
Signed-off-by: Fiete Ostkamp <Fiete.Ostkamp@telekom.de>