Make limit work in SQL Server 2008 by encoding it as TOP

[jbachhardie]

Description

Adapter now uses SELECT TOP X FROM... syntax when a limit exists without offset, rather than the OFFSET 0 ROWS FETCH NEXT X ROWS ONLY.

Motivation and Context

OFFSET/FETCH was added in SQL Server 2012 so depending on that syntax means that we couldn't have limit arguments in earlier versions despite limits without offset being supported (via TOP).

This PR closes https://github.com/findmypast-oss/mssql_ecto/issues/3

How Has This Been Tested?

Full test suite ran.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [x] All new and existing tests passed.

[shdblowers]

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