Composer version should be a valid constraint not ^2.1

[rfay]

Description

The current constraint here is invalid, composer has never supported a 2.1 branch (there is a 2.2). I'm not sure how this can work, but it seems that platform may try and fail to use 2.1, so just uses 2.8.x currently instead.

Related Issue

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Please drop a link to the issue here:

• Invalid composer version constraint ^2.1 #179

Motivation and Context

Use a valid composer version.

How Has This Been Tested?

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

Checklist:

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• I have read the contribution guide
• I have created an issue following the issue guide
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[rfay]

Platform docs say the same here: https://docs.platform.sh/languages/php.html#dependencies