Reduce heading size by inserting a "Deprecated functionality" super-section.

Refs #4618

docs/release-notes/2019/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 Changes anticipated to |Gromacs| 2019 functionality
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

docs/release-notes/2020/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 .. Note to developers!
    Please use """"""" to underline the individual entries for fixed issues in the subfolders,
    otherwise the formatting on the webpage is messed up.

docs/release-notes/2021/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 .. Note to developers!
    Please use """"""" to underline the individual entries for fixed issues in the subfolders,
    otherwise the formatting on the webpage is messed up.

docs/release-notes/2022/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 .. Note to developers!
    Please use """"""" to underline the individual entries for fixed issues in the subfolders,
    otherwise the formatting on the webpage is messed up.

docs/release-notes/2023/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 .. Note to developers!
    Please use """"""" to underline the individual entries for fixed issues in the subfolders,
    otherwise the formatting on the webpage is messed up.

docs/release-notes/2024/major/deprecated-functionality.rst

+Deprecated functionality
+------------------------
+
 .. Note to developers!
    Please use """"""" to underline the individual entries for fixed issues in the subfolders,
    otherwise the formatting on the webpage is messed up.