Bump .NET framework target to net462

[iamcarbon]

The final OS distribution supporting .NETFramework 4.5.2 has gone out of support. This bumps the library's minimum .NET Framework target to net 4.6.2.

[coveralls]

Pull Request Test Coverage Report for Build 3880616906

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 16 of 30 (53.33%) changed or added relevant lines in 5 files are covered.
• 4 unchanged lines in 2 files lost coverage.
• Overall coverage remained the same at 93.292%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Markdig/Syntax/Inlines/ContainerInline.cs	0	1	0.0%
src/Markdig/Syntax/MarkdownObjectExtensions.cs	3	4	75.0%
src/Markdig.Tests/TestDescendantsOrder.cs	0	12	0.0%

Files with Coverage Reduction	New Missed Lines	%
src/Markdig/Helpers/FastStringWriter.cs	1	99.07%
src/Markdig.Tests/TestDescendantsOrder.cs	3	0.0%

Totals
Change from base Build 3743767643:	0.0%
Covered Lines:	25916
Relevant Lines:	27171

---

💛 - Coveralls

[iamcarbon]

Ready for review.

[MihaZupan]

cc: @Kryptos-FR

[xoofx]

Thanks!