Composer.json: add link to security policy

[jrfnl]

Context

• Improve discoverability of security policy

Summary

This PR can be summarized in the following changelog entry:

• Improves discoverability of security policy in Packagist.

Relevant technical choices:

This is a new feature available since Composer 2.6.0, which was released a little while ago.

When this key is added, it will also show a link to the security policy on Packagist.

Refs:

• https://github.com/composer/composer/releases/tag/2.6.0
• https://github.com/composer/composer/pull/11271
• https://github.com/composer/packagist/pull/1353

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

• N/A

[coveralls]

Pull Request Test Coverage Report for Build 6985524925

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 48.608%

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Totals
Change from base Build 6925389201:	0.0%
Covered Lines:	1100
Relevant Lines:	2263

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💛 - Coveralls