Adjust .github files

[phil-davis]

Description

1. minor grammar error in CONTRIBUTING.md
2. don't need to talk specifically about "next major version" - we use semver and the next version number is whatever it will be. Don't need to mention backporting.
3. In issue_template.md don't need to mention about "your major branch..."

Motivation and Context

I noticed this stuff while adding these files to an app.

How Has This Been Tested?

N/A

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[codecov]

Codecov Report

Merging #36440 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master   #36440      +/-   ##
============================================
+ Coverage     65.05%   65.16%   +0.11%     
  Complexity    19802    19802              
============================================
  Files          1271     1271              
  Lines         74754    74754              
  Branches       1309     1309              
============================================
+ Hits          48629    48712      +83     
+ Misses        25739    25656      -83     
  Partials        386      386

Flag	Coverage Δ	Complexity Δ
#javascript	54% <ø> (ø)	0 <ø> (ø)	⬇️
#phpunit	66.38% <ø> (+0.12%)	19802 <ø> (ø)	⬇️

Impacted Files	Coverage Δ	Complexity Δ
...private/DB/MySqlSchemaColumnDefinitionListener.php	78.94% <0%> (+72.8%)	46% <0%> (ø)	⬇️

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