Lower-casing of scheme in UriComponentsBuilder breaks use of URI variables

[rstoyanchev]

Spring Security started seeing test failures after #33639 due to the scheme now being lower-cased. The OAuth2 redirect_uri in is parsed from {baseScheme}://... and on 6.2.0-SNAPSHOT, the scheme becomes {basescheme}, which does not match the template variable name. We need to revise the parsers to allow use of URI variables in the scheme.