Update checkstyle.xml

[Lasith650]

This will add below changes to checkstyle.xml,

1. Removing three properties(allowMissingThrowsTags, allowThrowsTagsForSubclasses, allowUndeclaredRTE) which were listed under JavadocMethod Module since they were removed from new versions of checkstyle. And this change is explained with this [1].
2. Adding allowMissingPropertyJavadoc property under a separate module named MissingJavadocMethodCheck. This property was previously listed under JavadocMethod module as allowMissingJavadoc. And this change is explained with this [2].
3. Module LineLength which was listed under TreeWalker module is now taken out from TreeWalker module. And this change is explained with this [3].
4. Removing FileContentsHolder module. And this change is explained with this [4].
5. Module SuppressionCommentFilter is taken inside to TreeWalker module. And this change is explained with this [5].

[1] https://github.com/checkstyle/checkstyle/issues/7329
[2] https://github.com/checkstyle/checkstyle/issues/6703
[3] https://github.com/checkstyle/checkstyle/issues/2116
[4] https://github.com/checkstyle/checkstyle/issues/3573
[5] https://github.com/checkstyle/checkstyle/issues/4714

[CLAassistant]

All committers have signed the CLA.

[afkham]

Please clean up your PR and send a proper one which has the relevant changes only. e.g. you have removed and added line length etc.

[omindu]

@afkham @Lasith650 Since these changes are not compatible with the checkstyle version (2.17) we use in wso2.parent v5.1. Builds of the components depending on parent v5.1 will break (parent v5.1 refers the master branch of the checkstyle.xml - https://github.com/wso2/carbon-parent/blob/v5.1/pom.xml#L193).

Caught this with one of our PR builders - https://github.com/wso2-extensions/identity-fetch-remote/runs/2945347866?check_suite_focus=true#step:5:1507.

Can we revert this for the time being ?