[gravitypdf-bot]

Summary

Bumps the plugin version from 6.18.0-RC6 to 6.18.0 for the final release. It's the same two-line change each release candidate got (most recently RC5 in #1979), and the 6.18.0 tag goes on this PR's merge commit.

• pdf.php: the Version: header and PDF_EXTENDED_VERSION go from 6.18.0-RC6 to 6.18.0.
• CHANGELOG.md: the gfpdf_process_merge_tags_in_submitted_rich_text line now says the filter is off for new installs, not off by default, since upgraded sites have it on.

RC6 was smoke-tested on a local site before this bump: every active PDF and template rendered, notification attachments, signed and unsigned URLs, PDF access rules, the admin screens, the Template and Font Managers, and the core font installer all worked.

Try it

1. Check out this branch and open Plugins in wp-admin: Gravity PDF shows version 6.18.0.
2. On a site that was on 6.17.x or a 6.18.0 release candidate, load any wp-admin page as an administrator: the gfpdf_process_submitted_rich_text_merge_tags network option is now 1.

Test plan

• CI passes
• After merging, tag the merge commit 6.18.0 and check the published release zip's pdf.php says 6.18.0

More info

• No other file references 6.18.0-RC6, and CHANGELOG.md already has its 6.18.0 heading.
• Moving to the final version is what runs the 6.18.0 upgrade routine for RC testers. 6.18.0-RC6 sorts below 6.18.0, so RC sites skipped it until now. Running the routine by hand on RC6 confirmed that 6.18.0-RC6 → 6.18.0 sets the rich text merge tag flag and 6.18.0 → 6.18.1 doesn't.