docs: tidy up security.txt and security.md file

[javorszky]

After a conversation with security folks in other NGINX projects, the recommendation was to completely remove the security.txt file and adopt the security.md file in the template repository.

Closes #1408

[thresheek]

This also needs a followup on the docs repo, since this file is deployed to the website: https://github.com/nginx/unit-docs/blob/main/Makefile#L58

[ac000]

• Rebase with master
• Commit message tweaks

$ git range-diff 54b7dce0...cc863e15
-:  -------- > 1:  63148a31 tools/unitctl: whitespace fixes
-:  -------- > 2:  5e8a6893 tools/unitctl: rename app -> apps, fix readme
1:  d80e3be6 = 3:  f2e05bc7 docs: remove security.txt file
2:  54b7dce0 ! 4:  cc863e15 docs: add SECURITY.md
    @@ Metadata
      ## Commit message ##
         docs: add SECURITY.md

    -    All new NGINX projects are created from the [template repository](https://github.com/nginxinc/template-repository)
    -    which has a SECURITY.md file in it. This PR adopts the file content.
    +    All new NGINX projects are created from the template repository which
    +    has a SECURITY.md file in it. This adopts the file.

    +    NOTE; We wrap the file around the 76-78 character mark for consistency
    +    and readability.
    +
    +    Link: <https://github.com/nginxinc/template-repository>
    +    Closes: https://github.com/nginx/unit/issues/1408
         Signed-off-by: Gabor Javorszky <g.javorszky@f5.com>
    +    [ Tweaked commit message - Andrew ]
    +    Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

      ## SECURITY.md (new) ##
     @@