Default versions should float the patch

[menehune23]

This buildpack's manifest.yml contains pinned default versions, such as php 7.4.23. We should change our defaults to float patches where appropriate, to help consumers build more secure apps (for instance, 7.4.23 has a CVE but the buildpack also provides 7.4.24).

[sophiewigmore]

This seems reasonable. I looked back at the history of the manifest.yml and I can't see anything that would indicate why this value needs to be hardcoded. We should do this.

[johnnyr0x]

Moving to icebox. Can pick up if it becomes an issue.