NEWS.md: reduce headings level [ci skip]

As the only h2 did not have its exclusively owned body text,
merged it to the first paragraph.  Then pulled up h3 and deeper
headings.

NEWS.md

 # NEWS for Ruby 2.8.0 (tentative; to be 3.0.0)
 
-This document is a list of user visible feature changes made between
-releases except for bug fixes.
+This document is a list of user visible feature changes since the
+ **2.7.0** release, except for bug fixes.
 
 Note that each entry is kept so brief that no reason behind or reference
 information is supplied with.  For a full list of changes with all
 sufficient information, see the ChangeLog file or Redmine
 (e.g. `https://bugs.ruby-lang.org/issues/$FEATURE_OR_BUG_NUMBER`).
 
-## Changes since the 2.7.0 release
+## Language changes
 
-### Language changes
+## Command line options
 
-### Command line options
-
-### Core classes updates (outstanding ones only)
+## Core classes updates (outstanding ones only)
 
 * Hash
 
@@ -23,9 +21,9 @@ sufficient information, see the ChangeLog file or Redmine
         * Hash#transform_keys now accepts a hash that maps keys to new
           keys.  [[Feature #16274]]
 
-### Stdlib updates (outstanding ones only)
+## Stdlib updates (outstanding ones only)
 
-### Compatibility issues (excluding feature bug fixes)
+## Compatibility issues (excluding feature bug fixes)
 
 * Regexp literals are frozen [[Feature #8948]] [[Feature #16377]]
 
@@ -39,13 +37,13 @@ sufficient information, see the ChangeLog file or Redmine
     If you interested in the maintain them, Please comment your plan
     to https://github.com/ruby/xmlrpc or https://github.com/ruby/net-telnet.
 
-### Stdlib compatibility issues (excluding feature bug fixes)
+## Stdlib compatibility issues (excluding feature bug fixes)
 
-### C API updates
+## C API updates
 
-### Implementation improvements
+## Implementation improvements
 
-### Miscellaneous changes
+## Miscellaneous changes
 
 
 [Feature #8948]:  https://bugs.ruby-lang.org/issues/8948