Correct copy/paste errors in bbp_get_topic_last_reply_* filters

[SergeyBiryukov]

Some bbp_get_topic_last_reply_* filters are misspelled as bbp_get_topic_last_topic_*:

• bbp_get_topic_last_topic_title​ should be bbp_get_topic_last_reply_title.
• bbp_get_topic_last_reply_permalink​ is correct, but misspelled in the docs as bbp_get_topic_last_topic_permalink.
• bbp_get_topic_last_reply_url​ is correct, but misspelled in the docs as bbp_get_topic_last_topic_url.

[netweb]

In 6244:

Topics: Fix misspelled bbp_get_topic_last_reply_* filters.

• Update bbp_get_topic_last_reply_title() to use bbp_get_topic_last_reply_title
• Includes a fallback for any plugins or themes that use bbp_get_topic_last_topic_title
• Fix PHPDocs @uses PHPDocs for bbp_get_topic_last_reply_permalink and bbp_get_topic_last_reply_url

Props SergeyBiryukov.
Fixes #3039.

[johnjamesjacoby]

In 6245:

Replies: Add @since and link to ticket from r6244.

See #3039.