fix: do not apply checkbox readonly background when disabled

[web-padawan]

Description

This was reported internall by @MatthewVaadin:

I was just working with checkbox states and noticed that if you set a checkbox to be checked (or indeterminate), disabled and readonly, you get a contrast that makes the check almost disappear

UPD: as discussed internally, let's make disabled state override the readonly to avoid contrast issue:

When it's disabled, the read only state is pretty much irrelevant.

Type of change

• Bugfix

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

Hi @web-padawan and @web-padawan, when i performed cherry-pick to this commit to 24.4, i have encountered the following issue. Can you take a look and pick it manually?
Error Message:
Error: Command failed: git cherry-pick b00d1b6
error: could not apply b00d1b6... fix: do not apply checkbox readonly background when disabled (#7733)
hint: After resolving the conflicts, mark them with
hint: "git add/rm ", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.5.0.beta2 and is also targeting the upcoming stable 24.5.0 version.