Update Support Level from Active to Stable

[jeffpaul]

Is your enhancement related to a problem? Please describe.

Once the following are completed:

• Custom https header based whitelisting #179

... we can then work to update the support level for this plugin from Active to Stable as we can generally consider the optimal feature set of the plugin complete. Tasks to complete this change:

• create a sticky forum post like https://wordpress.org/support/topic/read-this-before-you-post-64/
• make repo updates like Update Support Level from Active to Stable Ad-Refresh-Control#73

Designs

n/a

Describe alternatives you've considered

n/a

[jeffpaul]

DOTORG forum post here: https://wordpress.org/support/topic/read-this-before-you-post-68/