remove default session_duration value on access policies

[russellkerns]

Closes #2865

[github-actions]

changelog detected ✅

[jacobbednarz]

acceptance tests all passing

TF_ACC=1 go test ./internal/sdkv2provider -v -run "^(TestAccCloudflareAccessOrganization|TestAccCloudflareAccessPolicy_)" -count 1 -timeout 120m -parallel 1
=== RUN   TestAccCloudflareAccessOrganization
--- PASS: TestAccCloudflareAccessOrganization (7.52s)
=== RUN   TestAccCloudflareAccessPolicy_ServiceToken
--- PASS: TestAccCloudflareAccessPolicy_ServiceToken (12.03s)
=== RUN   TestAccCloudflareAccessPolicy_AnyServiceToken
--- PASS: TestAccCloudflareAccessPolicy_AnyServiceToken (10.84s)
=== RUN   TestAccCloudflareAccessPolicy_WithZoneID
--- PASS: TestAccCloudflareAccessPolicy_WithZoneID (21.04s)
=== RUN   TestAccCloudflareAccessPolicy_Group
--- PASS: TestAccCloudflareAccessPolicy_Group (10.58s)
=== RUN   TestAccCloudflareAccessPolicy_MTLS
--- PASS: TestAccCloudflareAccessPolicy_MTLS (11.22s)
=== RUN   TestAccCloudflareAccessPolicy_CommonName
--- PASS: TestAccCloudflareAccessPolicy_CommonName (10.87s)
=== RUN   TestAccCloudflareAccessPolicy_EmailDomain
--- PASS: TestAccCloudflareAccessPolicy_EmailDomain (10.21s)
=== RUN   TestAccCloudflareAccessPolicy_Emails
--- PASS: TestAccCloudflareAccessPolicy_Emails (10.61s)
=== RUN   TestAccCloudflareAccessPolicy_Everyone
--- PASS: TestAccCloudflareAccessPolicy_Everyone (10.15s)
=== RUN   TestAccCloudflareAccessPolicy_IPs
--- PASS: TestAccCloudflareAccessPolicy_IPs (17.27s)
=== RUN   TestAccCloudflareAccessPolicy_AuthMethod
--- PASS: TestAccCloudflareAccessPolicy_AuthMethod (10.57s)
=== RUN   TestAccCloudflareAccessPolicy_Geo
--- PASS: TestAccCloudflareAccessPolicy_Geo (10.95s)
=== RUN   TestAccCloudflareAccessPolicy_Okta
--- PASS: TestAccCloudflareAccessPolicy_Okta (10.22s)
=== RUN   TestAccCloudflareAccessPolicy_PurposeJustification
--- PASS: TestAccCloudflareAccessPolicy_PurposeJustification (15.54s)
=== RUN   TestAccCloudflareAccessPolicy_ApprovalGroup
--- PASS: TestAccCloudflareAccessPolicy_ApprovalGroup (11.09s)
=== RUN   TestAccCloudflareAccessPolicy_ExternalEvaluation
--- PASS: TestAccCloudflareAccessPolicy_ExternalEvaluation (10.47s)
=== RUN   TestAccCloudflareAccessPolicy_IsolationRequired
--- PASS: TestAccCloudflareAccessPolicy_IsolationRequired (17.11s)
PASS
ok  	github.com/cloudflare/terraform-provider-cloudflare/internal/sdkv2provider	218.773s

[github-actions]

This functionality has been released in v4.21.0 of the Terraform Cloudflare Provider.

Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!