bump spring_boot_version to 2.7.+

[bhou2]

This PR is to migrate Genie OSS SBN from 2.6.7 to 2.7.+ (i.e., bump spring_boot_version to 2.7.+). This migration is not backwards compatible (see https://github.com/spring-projects/spring-boot/wiki/Spring-Boot-2.7-Release-Notes).

In particular, for Genie: (1) H2database from 1.4 to 2.1.; (2) MySQL from 5.7 to 8.0; (3) PostgreSQL from 9.6 to 10; (4) Flyway from 7.x to 9.x. Accordingly, the related sql scripts / implementations, configurations, and dependencies need upgrades.

Tests have been done: (1) local gradle build; (2) genie build on github actions; (3) deployment and tests on personal genie stack.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (73aa7cc) 90.75% compared to head (351c821) 90.75%.

:exclamation: Current head 351c821 differs from pull request most recent head 3f861a1. Consider uploading reports for the commit 3f861a1 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1193 +/- ## ========================================= Coverage 90.75% 90.75% Complexity 3809 3809 ========================================= Files 467 467 Lines 14976 14976 Branches 1071 1071 ========================================= Hits 13591 13591 Misses 909 909 Partials 476 476 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.