Fix channel incompatibility between SDK and framework

[tosuke]

Output from acceptance testing:

$ MACKEREL_EXPERIMENTAL_TFFRAMEWORK=1 make testacc TESTS='TestAcc.+Channel'
TF_ACC=1 go test -v ./... -run TestAcc.+Channel -timeout 120m
?       github.com/mackerelio-labs/terraform-provider-mackerel  [no test files]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/mackerel        0.271s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/planmodifierutil        0.428s [no tests to run]
=== RUN   TestAccCompat_MackerelChannelResource_Email
=== PAUSE TestAccCompat_MackerelChannelResource_Email
=== RUN   TestAccCompat_MackerelChannelResource_Slack
=== PAUSE TestAccCompat_MackerelChannelResource_Slack
=== RUN   TestAccCompat_MackerelChannelResource_Webhook
=== PAUSE TestAccCompat_MackerelChannelResource_Webhook
=== CONT  TestAccCompat_MackerelChannelResource_Email
=== CONT  TestAccCompat_MackerelChannelResource_Webhook
=== CONT  TestAccCompat_MackerelChannelResource_Slack
--- PASS: TestAccCompat_MackerelChannelResource_Webhook (7.02s)
--- PASS: TestAccCompat_MackerelChannelResource_Email (7.35s)
--- PASS: TestAccCompat_MackerelChannelResource_Slack (7.38s)
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/provider        7.924s
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/typeutil        0.217s [no tests to run]
testing: warning: no tests to run
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/internal/validatorutil   0.958s [no tests to run]
2025/01/16 15:44:43 [INFO] mackerel: use terraform-plugin-framework based implementation
=== RUN   TestAccDataSourceMackerelChannelEmail
=== PAUSE TestAccDataSourceMackerelChannelEmail
=== RUN   TestAccDataSourceMackerelChannelSlack
=== PAUSE TestAccDataSourceMackerelChannelSlack
=== RUN   TestAccDataSourceMackerelChannelWebhook
=== PAUSE TestAccDataSourceMackerelChannelWebhook
=== RUN   TestAccDataSourceMackerelChannelNotMatchAnyChannel
=== PAUSE TestAccDataSourceMackerelChannelNotMatchAnyChannel
=== RUN   TestAccMackerelChannel_Email
=== PAUSE TestAccMackerelChannel_Email
=== RUN   TestAccMackerelChannel_Slack
=== PAUSE TestAccMackerelChannel_Slack
=== RUN   TestAccMackerelChannel_Webhook
=== PAUSE TestAccMackerelChannel_Webhook
=== CONT  TestAccDataSourceMackerelChannelEmail
=== CONT  TestAccMackerelChannel_Email
=== CONT  TestAccDataSourceMackerelChannelWebhook
=== CONT  TestAccDataSourceMackerelChannelNotMatchAnyChannel
=== CONT  TestAccMackerelChannel_Webhook
=== CONT  TestAccMackerelChannel_Slack
=== CONT  TestAccDataSourceMackerelChannelSlack
--- PASS: TestAccDataSourceMackerelChannelNotMatchAnyChannel (1.19s)
--- PASS: TestAccDataSourceMackerelChannelEmail (2.89s)
--- PASS: TestAccDataSourceMackerelChannelSlack (2.98s)
--- PASS: TestAccDataSourceMackerelChannelWebhook (2.98s)
--- PASS: TestAccMackerelChannel_Email (5.19s)
--- PASS: TestAccMackerelChannel_Slack (5.35s)
--- PASS: TestAccMackerelChannel_Webhook (5.51s)
PASS
ok      github.com/mackerelio-labs/terraform-provider-mackerel/mackerel 6.365s