CompatHelper: bump compat for MCMCChains to 7 for package test, (keep existing compat)

[github-actions]

This pull request changes the compat entry for the MCMCChains package from 6.0.4 to 6.0.4, 7 for package test.
This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry.
It is your responsibility to make sure that your package tests pass before you merge this pull request.

[github-actions]

Benchmark Report for Commit b6b72f5

Computer Information

Julia Version 1.11.5
Commit 760b2e5b739 (2025-04-14 06:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

|                 Model | Dimension |  AD Backend |      VarInfo Type | Linked | Eval Time / Ref Time | AD Time / Eval Time |
|-----------------------|-----------|-------------|-------------------|--------|----------------------|---------------------|
| Simple assume observe |         1 | forwarddiff |             typed |  false |                 10.0 |                 1.5 |
|           Smorgasbord |       201 | forwarddiff |             typed |  false |                720.1 |                34.5 |
|           Smorgasbord |       201 | forwarddiff | simple_namedtuple |   true |                419.3 |                52.2 |
|           Smorgasbord |       201 | forwarddiff |           untyped |   true |               1214.3 |                28.0 |
|           Smorgasbord |       201 | forwarddiff |       simple_dict |   true |               3218.0 |                23.7 |
|           Smorgasbord |       201 | reversediff |             typed |   true |               1433.5 |                30.6 |
|           Smorgasbord |       201 |    mooncake |             typed |   true |                937.6 |                 5.1 |
|    Loop univariate 1k |      1000 |    mooncake |             typed |   true |               5587.6 |                 4.1 |
|       Multivariate 1k |      1000 |    mooncake |             typed |   true |               1013.0 |                 8.8 |
|   Loop univariate 10k |     10000 |    mooncake |             typed |   true |              62412.8 |                 3.6 |
|      Multivariate 10k |     10000 |    mooncake |             typed |   true |               8584.6 |                10.1 |
|               Dynamic |        10 |    mooncake |             typed |   true |                130.1 |                13.2 |
|              Submodel |         1 |    mooncake |             typed |   true |                 14.1 |                 6.1 |
|                   LDA |        12 | reversediff |             typed |   true |                457.4 |                 5.7 |