Exclude ec2-subnet resource from phxdevops nuke config

Description

Adds ec2-subnet resource exclusion for phxdevops. We use default subnets for tests and they should not be deleted.

[x] Update the docs.
[x] Run the relevant tests successfully, including pre-commit checks.
[x] Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
[x] Include release notes. If this PR is backward incompatible, include a migration guide.
[x] Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Added ec2-subnet resource exclusion for phxdevops