Transition secrets-encryption from experimental to GA

[dereknola]

Background
The --secrets-encryption flag was introduced back as experimental in 1.17, with the secrets-encrypt subcommand introduced in v1.21.8+k3s1. After several months of stability, with the last big issue #5933 fixed in August, it is time to transition --secrets-encryption flag and the secrets-encrypt command from experimental to GA.

Describe the solution you'd like

Remove the experimental tags from K3s help messages and documentation.

Describe alternatives you've considered

Keep secrets-encryption forever as an experimental feature. This is detrimental to product support. We have have good testing coverage and usage of this feature. Its not going away.

Additional context

[ShylajaDevadiga]

Validated experimental tags from K3s help messages and all secrets encryption tests are working as expected

$ k3s server -h |grep secrets
   --secrets-encryption                       Enable secret encryption at rest