Rel-19 updates and editorials for v1.3.1

[jordijoangimenez]

• Header to V1.3.1
• Upgrade references to Rel-19
• 4.1. Review background and history (informative)
• 4.2. Add support for bands, TFI, CAS muting
• 4.3.2 Add text for Rel-18 and Rel-19

Editorial Guidelines from @rjb1000:

Citations:
All specifications referenced from the text proposal should be suffixed with the relevant reference number from clause 2, surrounded by square brackets.
Insert "TS" before each specification number cited.
Never specify a particular version of the TS in the normative text. This should be done only in clause 2, and then only if absolutely necessary as "Release 18", which means "the latest published V18.y.z".
The text proposal should propose changes to clause 2 as required to bring the references up to Release 18.
Clause headings with style "Heading 4" should be numbered explicitly.
Bulleted lists should use style B1 with the bullets entered manually with a hyphen followed by a tab.
Table caption should use style TH.
Table column headings should use style TAH.
Ordinary table data cells should use style TAL (left aligned), TAC (centre aligned) or TAR (right aligned).
There should be no blank lines anywhere in the normative text proposal except for one blank line after a table.
(The ETSI styles are designed not to require blank lines.)