Add 'release notes exist' validation to release process

[danudey]

Performs some validations on the release notes file before starting a release:

Release notes file does not exist:

❯ hack/release/release -create
time="2024-05-06T11:42:15-07:00" level=info msg="Running git command" cmd="/usr/bin/git describe --exact-match --tags HEAD"
fatal: no tag exactly matches 'e603ce9fe8158f05268f718fb2a45976b9e3478f'
time="2024-05-06T11:42:15-07:00" level=info msg="Running git command" cmd="/usr/bin/git rev-parse --is-shallow-repository"
false
time="2024-05-06T11:42:15-07:00" level=info msg="Running git command" cmd="/usr/bin/git rev-parse --abbrev-ref HEAD"
dfox_validate_release_notes_v3.28
time="2024-05-06T11:42:15-07:00" level=info msg="Running git command" cmd="/usr/bin/git describe --tags --dirty --always --abbrev=12"
v3.28.0-0.dev-567-ge603ce9fe815
time="2024-05-06T11:42:15-07:00" level=info msg="Current git describe" out=v3.28.0-0.dev-567-ge603ce9fe815
time="2024-05-06T11:42:15-07:00" level=error msg="Failed to create Calico release" error="release notes file is invalid: stat release-notes/v3.28.0-release-notes.md: no such file or directory"

Release notes file is empty (0 bytes):

❯ hack/release/release -create
time="2024-05-06T11:46:43-07:00" level=info msg="Running git command" cmd="/usr/bin/git describe --exact-match --tags HEAD"
fatal: no tag exactly matches 'e603ce9fe8158f05268f718fb2a45976b9e3478f'
time="2024-05-06T11:46:43-07:00" level=info msg="Running git command" cmd="/usr/bin/git rev-parse --is-shallow-repository"
false
time="2024-05-06T11:46:43-07:00" level=info msg="Running git command" cmd="/usr/bin/git rev-parse --abbrev-ref HEAD"
dfox_validate_release_notes_v3.28
time="2024-05-06T11:46:43-07:00" level=info msg="Running git command" cmd="/usr/bin/git describe --tags --dirty --always --abbrev=12"
v3.28.0-0.dev-567-ge603ce9fe815
time="2024-05-06T11:46:43-07:00" level=info msg="Current git describe" out=v3.28.0-0.dev-567-ge603ce9fe815
time="2024-05-06T11:46:43-07:00" level=error msg="Failed to create Calico release" error="release notes file is invalid: file is 0 bytes"