Add support for Terraform v1.4

[minamijoyo]

There are no changes in implementation, but some adjustments have been made to the test settings:

(1) Add terraform 1.4.0 to the test matrix
(2) Use a local filesystem mirror on CI
Starting with Terraform v1.4, the global plugin cache is ignored on the first terraform init. This makes caching in CI meaningless: hashicorp/terraform#32129, hashicorp/terraform#32494

To utilize the cache, we use a local filesystem mirror. Strictly speaking, the mirror is only available in Terraform v0.13+, but it is hard to compare versions in bash, so we use the mirror unless v0.x.
https://developer.hashicorp.com/terraform/cli/config/config-file#implied-local-mirror-directories

(3) Export TF_PLUGIN_CACHE_MAY_BREAK_DEPENDENCY_LOCK_FILE=true in CI
Starting from Terraform v1.4, launching terraform providers in the acceptance test has been failing more frequently with a text file busy error.

--- FAIL: TestAccMultiStateMigratorApplySimple (1.07s)
    multi_state_migrator_test.go:123: failed to run terraform init: failed to run command (exited 1): terraform init -input=false -no-color
        stdout:

        Initializing the backend...

        Successfully configured the backend "s3"! Terraform will automatically
        use this backend unless the backend configuration changes.

        Initializing provider plugins...
        - Finding latest version of hashicorp/null...
        - Installing hashicorp/null v3.2.1...

        stderr:

        Error: Failed to install provider

        Error while installing hashicorp/null v3.2.1: open
        /tmp/plugin-cache/registry.terraform.io/hashicorp/null/3.2.1/linux_amd64/terraform-provider-null_v3.2.1_x5:
        text file busy

After some investigation, I found Go's os/exec.Cmd.Run() does not wait for the grandchild process to complete; from the point of view of tfmigrate, the terraform command is the child process, and the provider is the grandchild process.
golang/go#23019

If I understand correctly, this is not a Terraform issue and theoretically should occur in versions older than v1.4; the changes in v1.4 may have broken the balance of execution timing and made the test very flaky. I experimented with inserting some sleep but could not get the test to stabilize correctly. After trying various things, I found that the test became stable by enabling the TF_PLUGIN_CACHE_MAY_BREAK_DEPENDENCY_LOCK_FILE flag was introduced in v1.4. This is an escape hatch to revert to the v1.3 equivalent of the global cache behavior change in v1.4.
hashicorp/terraform#32726

This behavior change has already been addressed in the previous commit using a local file system mirror, so activating this flag does not seem to make any sense. Even though I have no other reasonable solutions now, please let me know if anyone finds a better solution.