Remove GET request support from Saml2AuthenticationTokenConverter

[ngocnhan-tran1996]

I think we can define Boolean shouldInflate and method decode will be added

if (this.shouldInflate == null) {
	this.shouldInflate = HttpMethod.GET.matches(request.getMethod());
}

If not, this will break Saml2LoginBeanDefinitionParserTests and Saml2AuthenticationTokenConverterTests

Saml2LoginBeanDefinitionParserTests > authenticateWhenCustomAuthnRequestRepositoryThenUses() FAILED
    org.mockito.exceptions.verification.WantedButNotInvoked at Saml2LoginBeanDefinitionParserTests.java:323

Saml2LoginBeanDefinitionParserTests > authenticateWhenAuthenticationResponseValidThenAuthenticate() FAILED
    java.lang.AssertionError at Saml2LoginBeanDefinitionParserTests.java:213

Saml2LoginBeanDefinitionParserTests > authenticateWhenAuthenticationResponseValidThenAuthenticationSuccessEventPublished() FAILED
    java.lang.AssertionError at Saml2LoginBeanDefinitionParserTests.java:245

Saml2LoginBeanDefinitionParserTests > authenticateWhenCustomSecurityContextHolderStrategyThenUses() FAILED
    java.lang.AssertionError at Saml2LoginBeanDefinitionParserTests.java:228

Saml2LoginBeanDefinitionParserTests > authenticateWhenCustomAuthenticationManagerThenUses() FAILED
    java.lang.AssertionError at Saml2LoginBeanDefinitionParserTests.java:280

Issue: gh-17099

[jzheaux]

Thanks, @ngocnhan-tran1996! I've left some feedback inline.

[jzheaux]

Thanks, @ngocnhan-tran1996! This is now merged into main. Also in f73f253 I updated related authentication converter implementations. I also added migration docs.