Resolved more warnings, and marked more warning types as errors

[emmagarland]

Prerequisites

• I have added steps to test this contribution in the description below

If there's an existing issue for this PR then this fixes some of #15015

Description

Fixed various warnings where they are more straight-forward, including:

• IDE0270 null check can be simplified
• CS0105 namespace appeared already
• CS0162 remove unreachable code
• SA1028 remove trailing whitespace
• SA1106 no empty statements
• CS1570 malformed XML
• CS1572 corrected xml parameter
• CS1573 param tag added
• IDE0007 var not explicit
• IDE0008 explicit not var
• IDE0057 simplify substring
• IDE0074 compound assignment
• CA1825 array.empty

Suppressed the following warnings until code required is implemented:

• #pragma warning disable CS0162 // Unreachable code detected
• #pragma warning disable CS0618 // Type or member is obsolete

Updated the following projects to only list their remaining specific warning codes:

• Umbraco.Web.Website

Down to 3479 warnings.

Stopped at 50 files for review to avoid code getting too out of sync with new commits.

To test, ensure solution builds, tests and runs as expected. Worth scanning the files changes to ensure no likely issues I've not considered.

[github-actions]

Hi there @emmagarland, thank you for this contribution! 👍

While we wait for one of the Core Collaborators team to have a look at your work, we wanted to let you know about that we have a checklist for some of the things we will consider during review:

• It's clear what problem this is solving, there's a connected issue or a description of what the changes do and how to test them
• The automated tests all pass (see "Checks" tab on this PR)
• The level of security for this contribution is the same or improved
• The level of performance for this contribution is the same or improved
• Avoids creating breaking changes; note that behavioral changes might also be perceived as breaking
• If this is a new feature, Umbraco HQ provided guidance on the implementation beforehand
• 💡 The contribution looks original and the contributor is presumably allowed to share it

Don't worry if you got something wrong. We like to think of a pull request as the start of a conversation, we're happy to provide guidance on improving your contribution.

If you realize that you might want to make some changes then you can do that by adding new commits to the branch you created for this work and pushing new commits. They should then automatically show up as updates to this pull request.

Thanks, from your friendly Umbraco GitHub bot 🤖 🙂

[georgebid]

Nice "Chore" PR @emmagarland, I'll aim to take a look at this one soon 😄

[emmagarland]

Fab, thanks @georgebid!

[georgebid]

Hey, @emmagarland I've checked through this one + built and all looks good to me! Therefore I have gone ahead and merged it 🚀

[emmagarland]

Amazing @georgebid, thankyou!