Consider supporting the last two versions of each runtime in the latest images

[dls314]

Describe the bug The aws/codebuild/standard:6.0 image only supports the latest version of each runtime; this means that upgrading a CodeBuild job to this image version has to be done in lock-step with upgrading the project buildspec.yml files and project source files. It would be far nicer to have support that overlaps older images so that upgrades can be staggered and don't have to be done in lock-step.

Additional context The lock-step hits me when asking infrastructure teams to maintain CodeBuild/CodePipeline definitions used by development teams. Development teams using NodeJS 14 have to plan closely coordinated work with infrastructure teams who can't upgrade infrastructure to enable NodeJS 16 without breaking development because there is only one version of the runtime supported in the image.