Remove end of life ubuntu versions in CI (#10107)

* tests: replace ubuntu16 with ubuntu20

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: replace ubuntu18 with ubuntu20

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* docs: update docs to remove support for ubuntu 16 and 18

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* molecule: upgrade ubuntu versions

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* vagrant: upgrade ubuntu versions

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: cleanup ubuntu{16,18}

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

* tests: increase ubuntu22 ram to allow molecule creation

Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>

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Signed-off-by: Arthur Outhenin-Chalandre <arthur.outhenin-chalandre@proton.ch>