Summary:
Steps:
1. Create a sequence in PG11 with cache 1 (every `nextval` requires an RPC)
2. Take some values
3. Upgrade and take more values from the different tservers
4. Finalize upgrade and take more values
5. Create a new sequence in PG15, also with cache 1, and take values from it
Jira: DB-13374

Test Plan:
Jenkins: test regex: .*Pg15UpgradeTest.*

```
./yb_build.sh --cxx-test pg15_upgrade-test --gtest_filter Pg15UpgradeTest.Sequences -n 20
```

Reviewers: tfoucher, fizaa, hsunder

Reviewed By: tfoucher

Subscribers: yql

Differential Revision: https://phorge.dev.yugabyte.com/D39078