Latest installer release based on old Mbed CLI and GCC versions

On the Mbed website, all new users get directed towards this page to install Mbed CLI.
https://os.mbed.com/docs/mbed-os/v5.14/quick-start/offline-with-mbed-cli.html
For Windows users, this links to the release page on this repo. At the time of this ticket, the latest release is based on Mbed CLI and GCC that were in use in December of 2018 (a full year ago).

If cloning the latest Mbed OS version, this results in the following concerning warning.

```
[Warning] @,: Compiler version mismatch: Have 6.3.1; expected version >= 9.0.0 and < 10.0.0
```

Please create a new installer release with latest updates to the version of Mbed CLI and GCC associated with the latest Mbed OS release. Currently 5.14. Soon to be 5.15.

Note this is the same issue that was highlighted in https://github.com/ARMmbed/mbed-cli-windows-installer/issues/28, which was closed without explanation.

Temporary workaround for users that find this issue -

1. Update mbed cli with the following command
```
pip install -U mbed-cli --user
```

2. Update GCC
Download and install latest v9 at
https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads

Start with the Windows installer release page linked in the issue and compare its bundled Mbed CLI and GCC versions with the Mbed OS 5.14 requirements and the reported compiler warning. Review the prior issue mentioned for context. Done means a new installer release is available with current compatible versions and no version-mismatch warning for a fresh Mbed OS checkout.

Written by the indexing model from the issue text.