[Security] Bump vaadin.version from 8.1.6 to 20.0.3

[dependabot-preview]

Bumps vaadin.version from 8.1.6 to 20.0.3.
Updates vaadin-bom from 8.1.6 to 20.0.3 This update includes security fixes.

Vulnerabilities fixed

Sourced from The GitHub Security Advisory Database.

Stored cross-site scripting in Grid component in Vaadin 7 and 8 Missing variable sanitization in Grid component in com.vaadin:vaadin-server versions 7.4.0 through 7.7.19 (Vaadin 7.4.0 through 7.7.19), and 8.0.0 through 8.8.4 (Vaadin 8.0.0 through 8.8.4) allows attacker to inject malicious JavaScript via unspecified vector.

• https://vaadin.com/security/cve-2019-25028

Affected versions: >= 8.0.0, < 8.8.5

Sourced from The GitHub Security Advisory Database.

Timing side channel vulnerability in UIDL request handler in Vaadin 7 and 8 Non-constant-time comparison of CSRF tokens in UIDL request handler in com.vaadin:vaadin-server versions 7.0.0 through 7.7.23 (Vaadin 7.0.0 through 7.7.23), and 8.0.0 through 8.12.2 (Vaadin 8.0.0 through 8.12.2) allows attacker to guess a security token via timing attack

• https://vaadin.com/security/cve-2021-31403

Affected versions: >= 8.0.0, < 8.12.3

Updates vaadin-maven-plugin from 8.1.6 to 20.0.3

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[dependabot-preview]

Superseded by #124.