Update command help text for state migrate command

[SarahFrench]

Minor change to the help text.

1. Making the -upgrade flag's description more specific (you can only upgrade a provider used for PSS via this command)
2. Adding default value info to the two lock-file flags.

The lock file flags have an equivalent function to the -state-provider-lock flag in #38461 and when that flag isn't supplied Terraform will look -optimistically- at the .terraform.lock.hcl file in the working directory for a usable lock,

See this test case in TestInit_stateStore_newWorkingDir_inAutomationProviderApproval:

terraform/internal/command/init_test.go

Line 4763 in 2878910

t.Run("a state store provider downloaded via HTTP can be automatically approved if it already exists in the .terraform.lock.hcl file", func(t *testing.T) {

Target Release

1.16.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.