[release/9.0-staging] Use alternative format string specifier to ensure decimal point is present

[github-actions]

Backport of #111254 to release/9.0-staging

/cc @jkoritzinsky

Customer Impact

• Customer reported
• Found internally

ILDasm (9.0) outputs the wrong format in float64(...) and float32(...) for double and float values without decimal places. The decimal point is missing.
ILAsm interprets these as int64 and therefore casts them to double or float values, which leads to incorrect values.

Regression

• Yes
• No

[If yes, specify when the regression was introduced. Provide the PR or commit if known.]

Regression from .NET 9

Testing

[How was the fix verified? How was the issue missed previously? What tests were added?]

Manual verification

Risk

[High/Medium/Low. Justify the indication by mentioning how risks were measured and addressed.]

Low risk. Impacts ildasm usage only and the fix has been validated.

IMPORTANT: If this backport is for a servicing release, please verify that:

• The PR target branch is release/X.0-staging, not release/X.0.

Package authoring no longer needed in .NET 9

IMPORTANT: Starting with .NET 9, you no longer need to edit a NuGet package's csproj to enable building and bump the version.
Keep in mind that we still need package authoring in .NET 8 and older versions.

[dotnet-policy-service]

Tagging subscribers to this area: @JulieLeeMSFT
See info in area-owners.md if you want to be subscribed.

[jeffschwMSFT]

lgtm. we will take for consideration in 9.0.x

[jeffschwMSFT]

@jkoritzinsky can you take a look at the CI failures? we can merge when ready

[jkoritzinsky]

/ba-g all failures are known