releasenotes: Clarify kernel version used

Recently people have been asking which kernel version is shipped on our
releases, and pointing them to the pkglist of the images to find the
right stanza didn't seem great.

Let's make that bit of information explicit in the recent release notes.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

content/release/v2019.5/releasenotes.md

@@ -16,7 +16,7 @@ Apertis is committed to maintaining the v2019 release stream until June
 2021.
 
 This Apertis release is built on top of Debian Buster with several
-customisations.
+customisations and the Linux kernel 4.19.x LTS series.
 
 Test results for the v2019.5 release are available in the following test
 reports:

content/release/v2020.3/releasenotes.md

@@ -15,7 +15,7 @@ v2020 [release flow]( {{< ref "release-flow.md#apertis-release-flow" >}} ).
 Apertis is committed to maintaining the v2020 release stream until September 2021.
 
 This Apertis release is built on top of Debian Buster with several
-customisations.
+customisations and the Linux kernel 5.4.x LTS series.
 
 Test results for the v2020.3 release are available in the following test
 reports: