[AS] Fix issue with disk size validation

[lego963]

Summary of the Pull Request

Decrease minimum threshold of SYS disk size validation
Resolves: #1083

PR Checklist

• Refers to: Why can't the disk size for as_configuration be lower than 40GB? #1083
• Tests added/passed.
• Documentation updated.

Acceptance Steps Performed

=== RUN   TestAccASV1Configuration_basic
--- PASS: TestAccASV1Configuration_basic (35.76s)
=== RUN   TestAccASV1Configuration_publicIP
--- PASS: TestAccASV1Configuration_publicIP (36.54s)
=== RUN   TestAccASV1Configuration_invalidDiskSize
--- PASS: TestAccASV1Configuration_invalidDiskSize (8.50s)
=== RUN   TestAccASV1Configuration_multipleSecurityGroups
--- PASS: TestAccASV1Configuration_multipleSecurityGroups (53.97s)
PASS

Process finished with the exit code 0

[otc-zuul]

Build succeeded.

• golang-make-test : SUCCESS in 9m 42s
• golang-make-vet : SUCCESS in 8m 09s
• goreleaser-build : SUCCESS in 8m 39s