Alter meta robots directive on closed support threads

[jonoaldersonwp]

The meta robots tag on closed support threads is currently set to noindex, nofollow. This should be changed to noindex, follow, to allow crawlers to follow internal links.

E.g., See ​https://wordpress.org/support/topic/its-working-i-think/, ​https://wordpress.org/support/topic/best-plugin-454/

[Otto42]

We don't want robots crawling internal links for 2+ year old posts. Support posts that old don't have much value.

[jonoaldersonwp]

We want them crawling links back into the rest of the site, through the navigation. We definitely want to open this up. These posts are linked to, and being crawled anyway.

[dd32]

In 8044:

Support Forums: Tweak the robots tag to noindex,follow for old (>1 year) topics, and add it to closed topics (>6 months) without replies.

Fixes #4025, #4027.