Data-only migration fails on PostgreSQL-dialect target

## Expected Behavior
Migration succeeds.

## Actual Behavior
Migration throws an error:
```
can't migrate database: error while validating existing database:
can't read spanner schema: error trying to read and convert spanner schema:
couldn't get indexes for table public.test_table: couldn't read row while fetching interleaved tables:
spanner: code = "InvalidArgument", desc = "Statements with DISTINCT clauses are not supported"
```

`SELECT DISTINCT` is not currently supported with PostgreSQL dialect databases:
https://cloud.google.com/spanner/docs/reference/postgresql/query-syntax#select

Also, this part of the error message, `couldn't read row while fetching interleaved tables`, seems to be a copy-paste error.
[Error line](https://github.com/GoogleCloudPlatform/spanner-migration-tool/blob/v3.1.1/sources/spanner/infoschema.go#L350) in `GetIndexes()` vs [error line](https://github.com/GoogleCloudPlatform/spanner-migration-tool/blob/v3.1.1/sources/spanner/infoschema.go#L396) in `GetInterleaveTables()`.

## Steps to Reproduce the Problem

1. Create a PostgreSQL database with one table and one index (indexes don't really matter, but for the purposes of this error we'll include one)
1. Run a schema-only migration from PostgreSQL to a PostgreSQL-dialect Spanner database
1. Restart SMT
1. Run a data-only migration from PostgreSQL to the database created before

## Specifications

- Version: v3.1.1
- Platform:
```
# Server running SMT
> uname -a
Linux 6.1.0-11-cloud-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.38-4 (2023-08-08) x86_64 GNU/Linux

> go version
go version go1.21.0 linux/amd64

# Browser
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```