Dependency updates

[rsplithof]

Description

Breaking

• Dependency updates for:
  • php from 7.1 to 7.3
  • guzzlehttp/guzzle from ~6.0 to ~7.3
  • psr/log from ^1.0 to ^1.1

Changed

• Updated dev dependencies.

Motivation and context

Keep the repository updated with new standards.

How has this been tested?

• Affected unit tests are updated.
• Tested on test-api.exonet.nl

Release

## [v3.0.0](https://github.com/exonet/exonet-api-php/releases/tag/v3.0.0) - 2021-10-19
[Compare v2.4.1 - v3.0.0](https://github.com/exonet/exonet-api-php/compare/v2.4.1...v3.0.0)
### Breaking
- Dependency updates for:
    - `php` from `7.1` to `7.3`
    - `guzzlehttp/guzzle` from `~6.0` to `~7.3`
    - `psr/log` from `^1.0` to `^1.1`

### Changed
- Updated dev dependencies.